One of my favourite pranks in L4D is: When the host switches cheats on, I bind keys to spawning tanks and witches, and either 1. crash the server by overspawning, or 2. cause a lot of screaming, cussing and swearing to start in the LAN shop.
Incident #1: My teammate told me that the opposing team would switch cheats on when they played as Infected and spawn tanks to kill us all. Fair enough. So I said that I would spoil the game by binding keys to spawning tanks AND witches, and then I would spawn so many that the host would simply crash.
My teammate also said to set "noclip" on as well. That allowed us to fly out to the destination saferoom. And since tanks can't break down saferoom doors..I just closed the door and spawned as fast as my fingers could spam the keyboard. =D We crashed soon after that.
Incident #2: Guy A is sitting beside me playing VS. I notice that the host switches on the cheats when he needs to. Guy A turns around and comments something to his friend B - and I realised that he's the host. So before I joined the game, I bound my keys once more, and when I could finally join the game, I simply waited till B switched cheats on, and suddenly there were 10 witches and tanks in the distance, with 5 witches spawning nearby. I was insta-incapped, but guy A was just so damn stunned.
B quit the game asap, and then came over. A asked why he spawned so many tanks and witches so fast, and B said that he didn't, and must have been coz they realised that cheats were on and immediately spawned so many.
I was hiccuping with laughter. Srsly damn funny. XD
