The comment notification FAQ does not mention that notifications will be stripped from comments in some cases.
This is confusing because the line length limit takes effect after removal of notification, so a user may have written:
@SomeUser: OK!! (15 chars) and be told that it's not long enough, because
OK!! is only 4 chars.
It also does not mention that you may no longer notify more than one person per comment (unless there is a backtick in the comment). [covered here]
This documentation must be updated — a passing comment in some bug answer somewhere on meta is not good enough.