For things which are definitely broken, but where you want to not worry about it for the moment.
Use FIXME to flag something that is bogus and broken.
For useful features, optimizations or refactorings that might be worth doing in the future.
TODO used to justify not completing code at the time it is first written.
Use XXX in a comment to flag something that is bogus but works.
For things that require more thought and that are arguably broken.
An XXX demands attention and is not intended to stay in the code long. We may later decide that no problem exists. Or we may need to document unexpected behavior. If code is broken, we do not expect it to stay that way. We mark the problem immediately so that it is not forgotten.
BUG [ID бага] – обозначает, что в коде есть известный баг, который ещё не исправлен. REVIEW BROKEN HACK
Говорит о том, что код является мудрёным и нужно хорошо подумать прежде чем что-то в нём менять.