Blizzard said:Try this? http://forums.steampowered.com/forums/showthread.php?t=1048109
(except I dunno which folder you'd have to delete if verifying integrity fails)
Make sure you back up your game saves if you delete local content for games, though. It might delete them without warning you.
I tried to verify the content of the games and of the SDK. Still doesn't work.
Since it happens with all Source-based content, I should delete the Source files under the steamapps folder or at least the SDK files under my username folder (which I don't think it will work, but still...)? Because my internet connection sucks, and downloading all those things again would be a pain in the ass.
Update: Now it works. Just deleted the content under the <username> folder of HL2, EP1, EP2 and TF2. Thanks for the help, Blizzard.