Sometimes you can get away with a minor flaw, especially if it fits your character concept and the traits you've invented are more wimpy than normal.

Be careful not to invent a flaw that ruins the fun for other players.  While its certain that being obnoxious would count as a flaw for your PC, it is also quite certain that such a flaw would grate on other players nerves.  Other players are also going to count on your character to pull his or her own weight.  A flaw like "Can't keep a secret" might be good for comic relief, but it may also keep the other players from having the sort of game they look forward to.  Likewise, the "loner" character concept is almost certain trouble when playing a character in what is essentially a social pastime.  Think of any play you have ever seen, if the cast included more than one person, chances are that none of the characters was a loner.

If your PC's flaw (or any other part of your PC) is a pain in the butt, you take full responsibility for the trouble it causes to the other player.  Saying you are playing "in character" doesn't absolve you, because you invented the character in the first place.