Ha! Well, as a raid leader, it's usually extra bad form when I wipe a raid, but I've done it several times! Fortunately, my guild is forgiving, since I'm normally a little better than that.
Let's see . . .
In Tacvi, I repeatedly warned everyone not to use F8 targeting in Tunat's room (fake one) so they wouldn't hit the healing mobs around him. Right as we go in, I had to buff a warrior (ADD little bugger that he was, he didn't bother to check buffs before the fight), and when I used my mouse to click back on Tunat, I didn't notice that my target had switched to one of the healing mobs instead . . . Everyone running up got savaged as heals and bard song had their effects, hehe.
I wiped us on Vish twice, too! I got distracted and missed spotting an egg once, just recently. The other time was a mis-target (mob apparently memblurred, ran partway through me, then ran back when re-agro'd, but it apparently ran through me just as I targeted vish with my mouse and I never noticed.
(Good rule of thumb to practice there--always doublecheck your target window before nuking if you are in a situation with multiple mobs.)
Then there was the time I trained our newbie warrior trial with see invis mobs while he was afk at a zoneline. He was on the can, and tried to get back when he heard things beating on him and zone . . . just missed making it, too! Got there just in time to turn around, and . . . Loading, please wait! (I am an evil leezard.)
Everyone messes up some time. If it's a valid mistake and not someone just not paying attention, I generally don't read people the riot act on our raids. If they screw up from not listening though . . . I get testy! Most people don't do it twice! We only raid 3x a week, so we have to stay top form to remain in current content with all the flagging stuff to do.
Chutoi