Oh, and there are SCHUFA and debt collectors. Let's say a clause is invalid, but they gave you a bill with a due date. You can't ignore it or you'll get a bad SCHUFA or someone from a debt collection agency will knock on your door. This happened to a friend of mine recently ;( Then it's again your problem to prove that you are right. And it's once again a bureaucratic hell.

What if it's money you've already paid that someone won't give you back (while free or paid legal advisers tell you you're right)? It all ends with "you have to go to court". Yes, there is a chance that the other side will settle out of court. But usually they don't because going to court is a very expensive and long process and they don't think you'll do it (and most don't). And then it's just endless bureaucratic hell.

And if it's about the apartment, you've probably paid a deposit. And if you ignore some of the clauses, they will probably try to get back at you and punish you by not returning the deposit (or part of it). From here - GOTO 1 ;(