Back in uni around 1995, and some students had managed to get a bunch of games into a hidden directory on the network, including Doom.
A friend stumbled across this somehow, and so we ended up sat in one of the computer rooms one evening, frantically zooming through Doom in cooperative mode.
Well. Semi-cooperative; we spent more time shooting each other than the enemies. But it all worked reasonably well, until we got to the Cyberdemon and exhausted all the rockets on each other.
At which point, we were left with an angry Cyberdemon and no way to take him down.
Eventually, I think we plinked him to death with our handguns - you got a free clip of ammo every time you respawned. But one of my funniest moments in video gaming came when my friend ran out of ammo and decided to leg it up to the cyberdemon and started punching it's knee.
At the time, it was merrily blasting away at me; when I ran out of bullets, it turned around, noticed this little thing buzzing around its knees, and lowered it's rocket launcher. And shortly, little bits of Doom Guy softly pattered down from the sky.
A proper cartoon-comedy moment :)
One other (and rarely mentioned) feature of Doom was that if an enemy was hit by a stray round from another enemy, the two enemies would then fight to the death, until/unless they saw you.
IIRC, Doom II had a fun level, which was essentially just a room with a Cyberdemon in it, surrounded by Barons. When you stuck your head around the door, the Cyberdemon would take a pot shot at you and hit the barons, which would then trigger a massive brawl; all you had to do was sit and watch while they tore each other to shreds...
So something of a precursor to the "Mass battle simulator" game on steam...