StoneWilson is right, It is not really a penalty, but if you rely heavily on one forum to do your posting and then you loose the signature and links from that site then you will feel the hit when it gets discovered. This is also dependant on the amount of links that you have from other sites. Make sure that you are spreading out your link profile in order to make it as strong as possible.
Google unable to penalize you with that. Unless you get banned then you will get penalize. I face this problem before. So make sure that you do not get banned by forum so you do not get penalize from Google as well. Becareful when you do posting.