It can happen, it's called jury nullification. There's some interesting reading in the background section of the wikipedia article on it: http://en.wikipedia.org/wiki/Jury_nullification#Background

It's a bit controversial whether jury nullification is an actual right or whether it's just a side-effect of the fact that juries can't be punished or held accountable for their verdicts. What's the difference? Well, for instance, judges tend to instruct juries only to consider the facts and not the law, and attorneys aren't generally allowed to discuss jury nullification in their closing arguments. On the other hand, the jury will rule however it decides, so lots of things from the merit of the law itself to sympathy for the victim or defendant will factor into their decision.

It's occasionally upheld by the supreme court, occasionally overturned. It's defacto legal because it's unprovable. Also, it's worth noting that it's not exactly a panacea - the two big (in terms of scale) uses of it in the USA were northern jurors refusing to convict those who sheltered runaway slaves under the fugitive slave act (good) and the southern jurors who acquitted lynchers (bad). US link is more helpful http://en.wikipedia.org/wiki/Jury_nullification_in_the_Unite...

Judges and lawyers hate it, but at worst it's grounds for dismissal from a jury, not a crime.

I don't think jury nullification applies except for in criminal trials and only where the jury finds in favor of the defendant. You can't have juries running around convicting innocent people just because they don't like the law (or the defendant).

Actually technically jury nullification also applies to civil cases. But the judge has the power to throw out such verdicts.

Yes, and it's called jury nullification. The jury did not do so therefore they did not feel the law was unjust.