LstratosHFHas there ever been a more game breaking bug?
Yeah, I mean actively there's an on-going exploit to lag out any server by spamming a single command.
In the past there were exploits to crash servers, the ability to create an infinite number of sentry guns, specflagging rendering CTF unwinnable, specnading rendering mass friendly fire in pubs a thing, infinite crits on multiple classes, infinite ubercharge duration, infinite bonk duration, intentionally create Level [insert any number here] sentry guns, at launch there was a way to get under basically every map in the game, and honestly I'm probably still forgetting several big ones.
e:
The Sandman rocket mine exploit was pretty bad too.