I hate that whenever I ask someone what their greatest weakness is, they respond with:
"I work too hard."
"I'm too detail-oriented."
"I don't know when to let go--I always want to maximize profits. I really should loosen up sometime."
"I get too emotionally invested in my work."
So I always qualify the question with a list of unacceptable responses.
But really, candidates need to lighten up and give honest answers. No one is perfect, nor do good interviewers expect perfection. Something that all great geniuses seem to share is an obsession over past mistakes. I remember hearing an interview with Michael Jordan where he gave a very honest assessment of his game relative to that of Scottie Pippen, noting that Pippen at that point in their careers was a better 3-point shooter and defender. If you obsess over your failures, you're less likely to repeat them.
Jordan was unafraid to fail and to confront his weaknesses. So he missed some game-winning shots (Game 1, first NBA Championship game against the Lakers, 1991). He made a lot of them, too (umm, every NBA Championship Game that mattered against the Jazz).
Timid boasting is unappealing also. If you're good at some things, don't be afraid to toot your own horn. But don't give yourself these sheepish backhand compliments.