Illegal cd-keys

It would either be removed or....nothing.

Unless YOU illegally acquired it.

Then you would probably be banned, if the admins found out.

that game will get deleted from your library and/or your account will be banned or suspended.

Id have to say if it was illegally acquired directly from steam they would know something and ban the person who used it.
Also if it was taken from a developer of some sort they would notify steam about it.

I activated Dirt 3 key when few million of those leaked (didn't believe it would actually work). Few weeks later the game got removed without any notification, no warnings, bans or suspensions. I bought the game a month later, works fine.

I came to this thread expecting illegal CD keys and I don't see any! Awwwwwwwwwwwwwwwwwwwwwwwwwwww

(What a relief.)

Overall, Steam support are pretty reasonable. They'd just remove the game. They can't prove in any way that the person who put it in stole/pirated/whatever the key themself (you could have gotten it as a gift or in a trade, for example), so with that in mind they err on the side of caution rather than instantly banning people for something that might not have been their fault in any way, which I think is good.

Ninjas would attack and make the game disappear from your library. However, there would be no evidence of ninjas. If it got to that point and you are positive that the game was legit, ask Valve about your dilemma and be sure to mention that ninjas stole your game.