I've seen hackers in virtual person in TF2 on a Valve server. Permanent crits and a text spamming script. I've also seen infinite jumps and invulnerability in Metroid Prime Hunters, and modded weapons which crashed the game in the first Borderlands. Resident Evil 5 also has some glitches and hacks for infinite healing items and ridiculous weapons.
Wut bout you, Flood? What games and what hacks have you seen?
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MW2. I don't need to say more. combat arms: If I had a dollar every time my team was killed from across the map with a single knife swipe, I'd be pretty freakin rich right now. Kuma War was riddled with hackers too.