Discussions here have highlighted a few different types of -50 penalties relating to a variety of factors. One has even acquired it's own acronym - BLOOP (Back Link Over Optimization Penalty) See [webmasterworld.com...] and here [webmasterworld.com...]
You need to give us a bit more detail on what you feel you did that got you to this point. Have you submitted a reconsideration request? There are several examples posted here as well as advice on how to word one.
Try searching with site:webmasterworld.com [keyword]