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you have pireted games that use steam to set up a multiplayer game.

Possible reasons:

1st wrong App-ID in your steam-files (wrong entry in txt or wrong dll) (unlikely)

2nd Or wrong App-ID in a Steamcrack for another Game (more likely)

3rd Error in Steamdatebase outside your responsibility (absolutely impossible ;-) )

from what I know Serious Sam HD: The Second Encounter have a demo, but i don't know how it is listed in game list or things like that

It's F2P game for multiplayer and It's appid is used in pirated games to bypass protections, like Trine 2 (or 1?) Demo before it was blocked. :D

LOL at the backpedaling here. It's pretty obvious OP

it may be some of the mods u downloaded to contain a keylogger and someone really loged in on your steam and played a pirate version of serious sam happened to me before not on steam for other games that has nothing to do with steam dld some mod for 1 game and that mod had a keylogger even my antivirus didnt detect