That's nothing: one server I was on had a teleport mod, where a player could save their spot and warp to it later. I saved before the round ended, and when the teams switched, I teleported into the enemy's spawn room as a heavy. Chaos and angry people ensued.
Then, I went into spectator mode, moved to an inaccessible area in the enemy spawn room, saved there, switched back to my team as an engineer, and started building stuff in there.
Finally, I teleported out of bounds as a sniper and shot at people. It was a party/gift hunt server, so nobody cared that I wasn't exactly playing fairly.