Back in Runescape before Member's became such a huge difference in gameplay, 2 friends I made in the game decided the 3 of us should do Dragon Slayer, then afterwards upgrade to members together all at once. We spent about 6 hours of gametime gearing up with all sorts of stuff, then one of the wiseguys, though I swear it wasn't me, suggested we do the combat triangle of Melee, Ranged and Magic (Back in Runescape 2004 you DIDN'T fight as range or magic) I was the melee guy and the friends grabbed a bow and a staff. We entered the cave together where the dragon (elvarg) was. It wasn't instanced like it is now, so all 3 of us engaged the dragon. She threw two fireballs back to back and 1-hit KO'ed both of my friends. I grabbed their stuff off the ground and just as I was running away, she blew a fireball at me and dropped me to EXACTLY 1 HP (back when health wasn't in the upper 1000's). We were all cussing and having a good time.
I miss Runescape.. :c
Oh, and the other moment would probably be in WoW when I, a level 55 warrior, got owned by a level 30 druid. This was back in 2007 I believe, and we couldn't hold our breath underwater for very long. I tried my hand at ganking this tauren, but he ran away as a cheetah and hoped into the water west of grom'gol, around that island with the elementals. I chased him and kept charging when I could, but he went to swim form and swam down. Stupidly, I followed him... I was at the bottom when I started running out of breath, so I started to swim up as fast I could. NOT SO FAST! He started spamming entangling roots on me and I drowned.
I didn't play WoW for a couple days afterwards.