Well, it does happen sometimes. It's very cheap and easy to file a patent reexamination. You just need to dig up some prior art, pay about $2000 and you're good

Well, it does happen sometimes. It's very cheap and easy to file a patent reexamination. You just need to dig up some prior art, pay about $2000 and you're good to go. You can even do this as a defense against an infringement allegation and if you're lucky the court will suspend proceedings pending the outcome of the reexamination. The hard part is knowing in advance what frivolous bullshit patent the trolls are going to throw at you.