Hey, I'm pretty guilty of some stupid stuff...
For example, I was running Temple of Qarn (Hard) as a WHM. By this time, I was pretty familiar with the place, and I was very confident in my abilities. We get to the first pull, and I realize something horrible. I had the wrong hotbar open. I was spamming virus instead of cure 2. The tank promptly died, and I felt like a failure. The tank then proceeded to pull the smallest amount of mobs ever. I was way too embarrassed to tell him I could handle it, seeing as to how I was so scrub earlier.
It didn't help I got a massive lag spike in the middle of the dungeon...
That said, I've avoided Temple of Qarn ever since.