Just sit right back and you'll hear a tale, a tale of a fateful trip...
I made room in my WC boost run for a guildy (a co-worker irl even). He was a complete loot hog and would slow me down to loot. Granted, this was his first toon on this faction so he needed money. That's fine but I'd keep chugging along and getting xp. He started to loot too far ahead and die. That's fine, I was on a hunter, no rez for you, bud. He always complained that he wanted it on free-for-all so he could get cloth drops and grey trash to vendor. That's fine. I'd sit there and wait between instance runs while he went to vendor his over stuffed bags (that I crafted for him). The agreement was that I'd still get to do a /roll for boss loot. He was a druid so he wanted a lot of the same gear as my shammies. One day he runs up to the boss (before I could switch it to Group Loot - which is how I handled the boss loot so I got a chance at it), and aggros the fight. Then after I save his ass, he loots the body and gets the boss drop without my getting to do any kind of roll on it....
Later I was running other guild mates through SM. He asked to come with and I reluctantly agreed. This meant I only got to only take 2 of my shammies. I didn't want to hear him complain at work, so I let him come with. He insisted on free for all. I told him flatly NO. I said I'm here for gear too. If you want to come for xp great, but don't get greedy about stupid trash. So we end up killing Doan. With group loot on things should have been fine, but somehow we ended back on FFA. I don't remember how, I think I disconnected or something and wasn't group leader anymore. Anyhow, he was on a warrior (has no toons over level 30 but he's more alt happy than anyone) and decided he wanted the epic tanking ring that dropped. I wanted it for any of my shamans. I was all enhancement at the time and it seemed like a great ring for any of my toons. He insisted I should only get to roll on it once (thereby robbing me of it really). Then there was an awesome shield that also dropped later (we did several runs through all of the SM's). The chapeau also dropped from the same fight. While telling his irl wife that yes, she could take the hat (again we somehow ended up in FFA), he "misheard" and took that to mean he could take the shield.
I never let him go into any instances with me again. Since he was used to being helped all the time (Hydraxis Alliance are pathetic like that, they can't solo-quest to save their lives) and this was his first time as a Horde he didn't know what to do without being hand-held while leveling. The rest of the guild were only leveling little alts, and were doing solo-quests between boosting runs. Well, he finally dopped the guild and unsubbed. All because he was a loot hog and no one wanted to take him with to do anything. Now I never put it on FFA and I always roll on all my toons. Lesson learned.