Jury Nullification
There is the concept of "Jury Nullification", where the jury knows that a defendant is guilty, but decides to acquit him anyway. It's not a well known concept, and quite a few judges/prosecutors/governments/etc., wish that it didn't exist (so much so that any discussion of it can result in a mistrial!). But, nevertheless, it does exist:
http://en.wikipedia.org/wiki/Jury_nullification
Of course, the problem is that there's no guarantee that any particular jury will even know of that concept, let alone apply it.
Dave