I have one major one, which I were I to post here, would more than likely offend a lot of people, so I'll just zip that.

The ones I can publish would be: playing in an ensemble or group situation where the instruments are out of tune with each other; singers with bad pitch (just a tad flat or sharp, enough to give me a sense of fingernails grinding on a blackboard); singers or instrumentalists who forward phrase everything, like "they can't wait to be on the road again"; drummers who can't keep time, and drag their ass and the rest of a band down with them -"while the world revolves around them"; inappropriate solos that don't work within the context of the song genre (i.e. doing an Eddie Van Halen or Hendrix type of solo, in the midst of a tender torch song or jazz standard). I'm sure I can think of more, but no doubt someone else, will post those thoughts for me.