The problem with clumsiness as a flaw is that, inevitably, the Mary Sue RPer controls when it 'kicks in' so to speak. In a situation where it wouldn't matter, or would be fortuitous, it will go off all the time, like dropping something during a social RP in town, or tripping and falling during a fight at just the right moment to avoid an otherwise unavoidable attack. In a situation where the clumsiness would mean something, like during a 'fight for life' situation, or a chase scene, the flaw is always completely forgotten. This is, actually, the problem with typical Mary Sue flaws in general; most Mary Sue characters ignore whatever flaws they add in, unless the flaw benefits them in some way at that moment. Mary Sue isn't just in the character itself, it's in the execution of that character.

While it is alright if you are attractive, or are among the best in your field at something, or have done work directly with the major canon characters in the world (I mean, if you are level 85 without having taken and performed at least one quest that involves a faction leader, you've been questing in all the wrong zones) the problem arises when those background and cosmetic factors overshadow your character actually having a real personality. If after an RP, the only thing you remember about a character is that they are in the top percentile of Shadow Priests, they have item level 400 epic breasts, and that Sylvanas is their older sister, that is a Mary Sue. That same character is not a Mary Sue if you encounter them, and find that their lisping speech and outrageous conspiracy theories about the Argent Dawn are what you remember after the RP is over.