Well, I just started 5-boxing last weekend so while I hope that these remain the stupidest things I do while multiboxing, I suspect that it's just the start of even stupider mistakes to come!

First, since I loved druids back when I played WoW solo, I rolled up a team of 5 druids and when I got to 16 I went to get the aquatic form quest. I think I looted at least my 7th or 8th bauble thing from underwater before I remembered that even though the quest timer gives you half an hour, once you loot the item it only lasts for a few minutes. I was very thankful when I realized that I could just buy the ability from the trainer instead...

Then, following the awesome RAF 1-60 TourGuide, I was doing the quest in Bloodmyst to "coerce" the secret message from Sunhawk elves. I looted it on my first kill, happily ran back to Blood Watch and I must have stared at the screen for five minutes wondering why only my main got a quest completed message. I guess the questgivers won't accept four characters pointing at the main and saying, "seriously, we did find it, he just gave it to you, right?"