I remember doing the City of Skulls once, on 0996 I believe, preparing to take on the Agony Elemental, already surrounded by a couple of it's younger brothers. I had a missile launcher on me, so I thought I'd empty that on the group (unaware of any fire resistances). When this wasn't getting me anywhere, I thought I'd rocketjump out of there with my last missile. Mistake number 2, since it doesn't have this alternate fire mode.
The third and final mistake was pressing alternate reload instead of alternate fire, which caused Doomguy to blissfully start loading 3 rockets into the bloody thing whilst surrounded by lost souls. Byebye Doomguy.
Another reaaallly pretty one was one of those A100 MSs runs, in which I was lucky enough to find the GCB as well. I'd taken cover behind a corner and was firing away at a Mancubus (betrayed by having INT2) which would be on the edge of my LOS had I attempted to face it head on. All was going well until I half missed the 'f' key on a shot and ran into the open instead of moving the fire cursor. Ate about 5 rockets in one second.
The beauty was that, because I'd already hammered in the entire path the fire cursor should take, I would move 1 square, take a rocket and suffer knockback, move another, etcetc. I was staring at my screen open mouthed as Doomguy appeared to dance himself to death :S