Damage Glitch: The game has different IDs for different attacks, and enemies can only keep track of two of those IDs at a time. By attacking with two different kinds of attacks and then another different kind before the invincibility time of the other two ran out, you can overload the enemy's invincibility and hit with one of the attacks on every frame. Which of the three attacks hits multiple times depends on their IDs. For some examples on how to perform it;
Jump and throw a cross, then divekick and whip. You might need to do delay the divekick to make it work.
Throw a cross or axe, hop forward and whip in the air, then dash cancel after landing and whip again. This also works with some spellbooks like Fire Cross, Fire Fist, and some others.
Summon a fairy with Summoning Tome + Bible, then charge the crushing stone whip, and when the fairy has the hammer raised, release the whip really close to the enemy. This is possibly the most damaging damage glitch in the game. The charged whip only hits with both types of its hitboxes if you're really close when you release it. This damage glitch won't work unless it's done with the crushing stone.
Ice Fist Glitch: If you can almost jump up to a ledge, the ice fist spell can prop you up a few pixels onto the ledge.
Door Glitch: When entering a room through a door, you have control of the character for one frame on the first frame of the new room or last frame of the old room. (It doesn't seem consistent which frame it is if any, so it probably depends on subpixels.) This can save time by dashing through or by charging the crushing stone whip early. More importantly, by backdashing into the old room, it can sometimes warp you somewhere. All of these involve dashing right after entering the room to the left. They don't work if you're entering the room on the left the first time so that the area name appears. Details on door warps given in the Out of Bounds section of the site.
First Jump Conservation: If you go off of a ledge while using a subweapon, the few frames that you can jump after going off of a ledge only start after the subweapon animation ends, which means you can save your first jump for later after going off of a ledge. This is mostly useful with the fist to enter Dracula's room with only a double jump. It can also be used instead of a launch to get the early platinum tip.
Candle Invincibility: After touching a candle stand you become invincible for a short while. This can often allow you to simply dash through enemies to save time over fighting or avoiding them.
Moon Physics: Some zips take you into a wall in the water room in Cavern A, which then takes you to Aqueduct B. If the water wasn't lowered by using the Floodgate Key, this gives Juste moon physics which opens up more possibilities for zips. It can also be triggered by zipping out from any of the other water rooms.
Ceiling Sink: Super jump with moon physics active and then whip on the first possible frame afterward so that you have the whip out while touching the ceiling. Then make the whip go limp so that you slowly go into the ceiling. If the ceiling is thin so that you don't need to go all the way inside it, you have a few more frames to whip and then you can finish going above the ceiling by jumping.
Box Fist Glitch: There are two rooms in the game which have boxes that can be pushed against stairs in them, both in Skeleton Cave A. When you push a box against stairs, it bounces back. To do this glitch, push a box as far as possible on the stairs so that the next touch will make it bounce back, then jump and use the fist toward the box while you're in air. This should make the box bounce back and push you down below the floor. In one of the rooms there is no ground to land on below so it helps to use the Aurora Ring to cancel the fist animation OoB.
Drawbridge Glitch: When Talos is chasing you in the autoscroller at the beginning, you can get hit so that you're damage stunned as the drawbridge raises, making you warp to Clock Tower A above the ball race room. An alternative is to jump and whip toward the drawbridge as it closes so that it zips you up, warping you to the same room but on top of the platform.
Old Relic Skip: It has to be your first visit to Sky Walkway (the textbox has to pop up): you trigger a cutscene glitch there using the Maxim cutscene as you immediately zip out of the room. You start the divekick left immediately after you get the map for the block shown in the vid (technically you can also start it shortly before mapping the block, but it's easier to know you're in the correct spot if you wait for the map to fill in). If you're too late with the divekick, the game will crash shortly after you see a white flash. You have to divekick for approximately 2:25 to reach the flags.
Talos Leveling: Killing Talos in particular with damage glitch when it will cause Juste to level up appears to cause a level up for every frame the damage glitch is active.
Cavern Wall Despawn: A way to despawn the Crushing Stone barrier to get access to the Warp Room in Luminous Cavern A. Enter the room from the bottom, and wait at the bottom for about 50 seconds (in the video I wait a little bit longer just to be sure). That's it, the barrier is now gone. The reason for this is the game spawning an Imp every few seconds. Eventually this caps the sprite limit, which in turn prevents the barrier (and anything else not yet loaded) from spawning. The Imps themselves then vanish as the game sort of collapses on itself.
Camera Lock: Zipping out of the bloody elevator room in Castle B before the elevator reaches the top causes the camera to be permanently locked in place. What this means for the game, is that whatever is outside of the camera view, doesn't exist: while the layout of the areas is exactly the same as before, enemies, doors, spikes and traps aren't there to block you, unless you can see them. So, if you carry this status to the Dracula-chamber hatch, you can just fall right in, instead of collecting any of Dracula's Relics. Maneuvering blind is pretty annoying, but since there's no enemies to worry about, it's all just memorization. Also, this same trick can be used to skip essentially any other door. Griffin's Wing is not required, you can alternatively use the blue ghosts and a divekick to get up on the platform. Camera lock can also be triggered by the Talos zip to Entrance A's save room at the start of the game.