The first was when I was healing Hellfire Citadel on my Worgen Druid. The pug had a baby dk. My first thought was "oh boy" cause new dk tank meant heavy healing.
First group the guy couldn't hold aggro to save his life. We barely survived after words one of the dps note he did not have Blood presence on. So ok maybe he just forgot big deal but it really got annoying when he says he had no clue what blood presence was. After several minutes of this I asked what his main was. He said it was a hunter. After several more minutes of trying to tell him what BP was he leaves. To this day I can't figure out whether he was serious or just a troll trying to waste our time and *%!! with us.
The second incident I was in a pug group doing heroic icc for the mount. now this after Cataclysm went retail. There was this 85 pally who was doing horrible dps barely half of what everyone else was doing. after we down putricide somebody looks at her gear and says she has a bunch of int gear on her. I take a look and she has a mix of int and str with an agility blue cloak. Her talents where all over the place in the ret tree and when I checked her armory profile she only had three prime glyphs and a major glyph. We then spent the next half hour going through her trees and gear telling her what to use and which glyphs to use. her guildie who was in the group promised to help her with her gear.
The third incident was in heroic SFK with a Lock who had a str neck, ring and sword. Never explained every time I asked him.