Nick collision
What happens when two IRC users with the same nickname meet after a netsplit and the network has to decide who keeps it.
During a netsplit, neither half knows which nicknames are taken on the other side. Anyone who wanted someone's name only had to connect to the right half and wait. When the servers rejoined, the early protocol had exactly one answer: KILL both users. That made a handy weapon — colliding someone off the network on purpose.
The 1990s brought two fixes. Timestamps (TS) in the server protocol let the network compare when each nick had been taken, so the older one survived and the newer one lost. IRCnet chose "nick delay" instead: a nickname that vanished in a split was locked for everyone for a while. Registration with NickServ covered the rest, on networks that, unlike EFnet, didn't refuse services on principle.