The list of things commonly misunderstood rules
1. You do not need trapfinding in Pathfinder to locate a trap. You only need it to disable a magical trap with disable device. Anyone can make a perception check to find a trap, even a magical one. (Aside: Trapfinding does not automatically find you traps. You have to be actively searching unless you have the Trap Spotter talent.) (Exception: To use a perception check for "symbol" spells such as Symbol of Death, the PC needs trapfinding.)
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 417
2. Bardic performance is a free action to maintain and does not inhibit any other actions the character may wish to perform while doing so, which includes bardic performances like dancing or acting, except if the character wishes to start another bardic performance, in which case the previously maintained performance immediately ends. Classes with inspire courage can also perform bardic performances even if they don't have ranks in the appropriate perform skill, but not necessarily other bardic performances.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 35-38
3. The following spells have 1 round cast time (note, NOT full round action): Enlarge Person, Reduce Person, Sleep, Infernal Healing, Silence, Deep Slumber, Summon Monster, Dominate.
4. You cannot "partial charge" in Pathfinder. You can charge as a standard action ONLY if you are denied having a full round action for that turn, such as a surprise round or being staggered. Doing so, you can only move up to your full speed instead of double full speed, and you cannot draw a weapon as a part of the standard action charge even with a +1 BAB or greater (Quick Draw can mitigate this).
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 198
5. You can take 10 any time you are not in a time of distraction or stress (like combat) on any skill. You may also take 20 on a skill (taking 20 times as long) to take a check if there is no penalty for failure. If there is a penalty for failure, that occurs when you take 20, you cannot take 20's on skills with no retries. Note: An exception to this is the Use Magic Device skill.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 86
6. For ranged and reach attacks: To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC). As well, for a burst spell that requires a reflex save (such as burning hands), if a creature has cover vs. the spell, that creature gets +2 to their Reflex save vs. the spell.
For melee attacks adjacent to you: When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target's square goes through a wall (including a low wall). When making a melee attack against a target that isn't adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.
Update: James Jacobs answered a question about having a reach attack and attacking around a corner when it's a large creature with reach (or an enlarged person for that matter) where they do not have cover when attacking around a cover in certain cases. Link.
Update 2: If a creature has cover and if more than half the creature is visible, it is partial cover, which is +2 AC instead of +4 AC. Whether this applies to soft cover or not is unclear, and I could see it being ruled either way.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 195-196
7. You cannot take ranks in the Fly skill without a natural means of flight or gliding. Creatures can also take ranks in Fly if they possess a reliable means of flying every day (either through a spell or other special ability).
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 96 (very last paragraph)
8. You can take swift actions during a surprise round or while staggered. You can take it in the surprise round because you can take free actions in the surprise round, and you can take swift actions whenever you can take a free action.
Sources: (Staggered: d20pfsrd, Paizo PRD, Core Rulebook pg. 568) (Surprise Round: d20pfsrd, Paizo Prd, Core Rulebook pg. 178) (Swift Actions: d20pfsrd, Paizo PRD, Core Rulebook pg. 188)
9. Most animal companions can only wear barding and neck slot items. The only exception to this would be an brownie, imp, lyrakien azata, or quasit familiar gained with the Improved Familiar feat. Wondrous Items work if it makes sense with anatomy. You can give them a magical ring if you purchase a Hand of Glory. Humanoid ACs, such as monkeys, apes, and imps, have all the slots a human would. Ioun stones do not work on companions that are the animal type.
Update: This still holds true in PFS despite the Animal Archive, however if you own the Animal Archive there is now a feat that can grant a companion/familiar an extra slot based on what its anatomy is.
Sources: (Animal Companions: Organized Play FAQ) (Ioun Stones: d20pfsrd, under the "Facts About Ioun Stones" heading.)
10. Wands do not reduce the casting time of spells that have a casting time greater than a standard action. Wands DO increase the casting time of any spell less than a standard action (I.E. move action, swift action, immediate action, free action) to a standard action. This rule also applies to scrolls.
Sources: (Wands: d20pfsrd, Paizo PRD, Core Rulebook pg. 496) (Scrolls: d20pfsrd, Paizo PRD, Core Rulebook pg. 490 (second to last paragraph))
11. The knowledge check to identify a monster using the appropriate knowledge skill is:
Common: 5 + CR
Uncommon: 10 + CR
Rare: 15 + CR
Making the check lets you know a "useful bit" about the monster. Each 5 by which the check is exceeded gives another bit of information. The chart shown for knowledge checks is misleading in this regard, as it is described in the text how knowledge checks truly work. (It should be noted that this is slightly up to GM discretion, however the vast majority of knowledge checks should fall in the "Uncommon" region. The book sites goblins as common and the tarrasque as rare, which are two extreme cases.)
Sources: d20pfsrd. Paizo PRD Core Rulebook pg. 100
12. Sneak attack, criticals, and flanking.
You can't sneak attack, critical, or flank:
Creatures with Ooze Traits
Creatures with Elemental Subtype
Creatures with Swarm Subtype
Incorporeal creatures without a ghost touch weapon
Creatures with the Amorphous special quality
Special cases:
Aeon subtype makes the creature immune to critical hits only.
Protean subtype gives the creature a 50% chance to ignore sneak attack or critical hits.
Unless specifically called otherwise, all other creatures, such as Undead, Animated Objects, Constructs, and Plants can be sneak attacked and affected by critical hits.
Sources: d20pfsrd. Paizo PRD. Bestiary pg. 306-313
13. When you have a 10 ft. reach, you do not threaten the square that is 2 diagonally from you because technically it is 15 ft. reach. However, if someone passes from the 15 ft. away square to the 5 ft. away square and would normally threaten for passing through a threatened area, they do (or vice versa moving the other way). If you do a combat maneuver that stops movement such as trip and succeed, they remain in the square they were previously.
Update: This has been overruled! You now threaten out 2 diagonals if you have reach!
Source: FAQ Ruling.
14. When using Use Magic Device to activate something, you do not automatically fail on a natural 1 (just like any other skill). If you do fail the check and roll a natural 1, then you cannot activate the item using Use Magic Device for 24 hours.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 109
15. You can voluntary fail a saving throw, but only versus a spell.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 217
16. You can make sunder attacks as part of a full-round attack action. (Note: as well as Disarm and Trip)
Sources: Pathfinder Core RPG FAQ
17. Soft cover is not an exception to the rule of providing cover for attacks of opportunity with reach attacks. In other words, if there's soft cover involved when determining attacks of opportunity, it acts the same as normal hard cover and does not allow an attack of opportunity.
Sources: (Reach attacks are ranged attacks and not making AoO's through cover: d20pfsrd, Paizo PRD, Core Rulebook pg. 195)
18. If you are prone, you can 5 foot crawl as a move action (not full-round action). This provokes attacks of opportunity.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 186
19. You cannot take 10 on spellcraft checks to identify magic items when using detect magic. Concentrating on a spell counts as strenuous activity.
Source: Link.
20. Spell trigger items, like wands, can be used by characters who have the spell in question on their spell list but cannot yet cast spells, for example rangers and paladins that are below level 4.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 458
21. If you cast a spell while grappled, you must make a concentration check (DC 10 + the grappler's CMB + the level of the spell you're casting) or lose the spell.
Update: This is different from before! The spell you cast can now contain somatic components.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 206
22. A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack. Likewise, if a creature's saving throw succeeds against a targeted spell, you sense that the spell has failed. You do not sense when creatures succeed on saves against effect and area spells
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 216
23. You can pull out two light or one-handed weapons in the time it takes to pull out one if you have the two-weapon fighting feat. If you have +1 BAB and the two-weapon fighting feat, you can pull out two such weapons as part of one move action.
Sources: d20pfsrd Paizo PRD. Core Rulebook pg. 187
24. Mindless creatures can be affected by certain illusion magic such as shadow spells (like shadow conjuration), figments (like silent image and mirror image), and glamers (like blur), but not patterns (like color spray) and phantasms (like phantasmal killer).
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 210
25. To mechanically benefit from a deity in PFS, you must be within one step of its alignment
Source: Organized Play Guide.
26. Slotting a wayfinder with an ioun stone takes away the ability to for the wayfinder to function as normal (i.e. it can't use light anymore). The ioun stone still grants its normal benefit. Cracked/flawed ioun stones can't be slotted for an additional benefit; only normal ones.
Sources: d20pfsrd-ioun stones. d20pfsrd-wayfinders.
27. An ability penalty (such as from exhaustion or touch of idiocy) can not reduce an ability score below 1.
Sources: d20pfsrd. Paizo PRD. Core Rulebook pg. 555
28. Ranger animal companions get the favored enemy and favored terrain bonuses of the ranger.
Sources: d20pfsrd., Paizo PRD. Core Rulebook pg. 66
29. Creatures with hardness? Exact quote from Paizo Blog: "When a creature with hardness sustains damage, subtract its hardness from the damage dealt. The rules for halving damage, doubling damage, dealing damage with ineffective tools, immunities, and the like only apply to damaging inanimate objects." So, if it has hardness but is not inanimate, you subtract hardness but the damage is not halved.
Source: Paizo Blog.
Original Source: http://www.georgiapfs.org/forum/index.php?topic=986.0