Recurrent errors, and how to fix them:
Package won't install, "could not be found". Try only installing for single user, instead of all users. This may be due to having TexStudio/whatever open with a lock on the package database whilst trying to install it.
"undefined control sequence" in a previously working bibliography (i.e. not because there's an actual typo). Delete the .bbl file and recompile.
Bibliography not working, even though options in usepackage, addbibresource, and reference are all correct. Check your Build & View command (in TexStudio options or wherever) is: TeX-make bibliography-TeX.
E.g. XeLateX-Biber-XeLateX
Check that the command shortcutted as "Biber" is in fact "Biber [filename]" - in TexStudio, you can write this as "Biber %", and TexStudio will replace % with the current file.
Run the "bibliography" command separately, and check that you get a .bbl file. This may be enough to fix it.
I got the line "ntry]nyt/global" printed mysteriously at the beginning of my Xelatex document.
It goes away when I change \usepackage[disable]{todonotes} to \usepackage[colorinlistoftodos]{todonotes}.
"Missing \endcsname inserted. \chapter". Error vanishes if \chapter{} command is deleted.
Doesn't seem to stop anything compiling