Litigation and in particular post-grant validity challenges at patent offices provide an important mechanism to correct the granting of weak patents. However, only if the (expected) private gains from challenging a granted patent right exceed the costs of a challenge, will there be validity challenges. Even then firms may refrain from challenges, if they anticipate that many others will also benefit from the revocation of a weak patent – this is a public goods problem. Finally, as more firms are caught up in patent thickets, challenges to weak patents may become too costly, if they invite counter-challenges. Using data on opposition proceedings against patents granted at the European Patent Office (EPO) our study points to two fundamental weaknesses of post-grant review: in technical fields characterized by low concentration of patent ownership, the likelihood of correcting erroneous grant decisions by third-party opposition is low. Secondly, in fields with a large numbers of mutually blocking patents incentives to oppose patents are sharply reduced.  This shows that the social benefits of post-grant review are currently concentrated in technical fields with discrete technologies and strong ownership concentration.