A few days ago a prankster infected Gary's Mod with a worm via a .dll file. I won't go into too much detail but suffice it to say it was just a prank and nothing overly malicious was done it seems, Valve and Gary Newman (the mod's creator and namesake) patched within hours of the issue arising.
They still advise caution and being careful about what you download.
You can check out PC Gamers story here.