In Belgium I followed a cource on hunman resource law. What they said to me (as employer, not as employee) was that you can basically write everything in a contract, but that does not make it legal. Not only could it mean that a part of the contract can be trown out, but the whole contract can be trown out AND the law will be even more inclined to side with whatever the emplyer tells.
On the other hand, if the people do not complain within a year after termination, you are good.
Th ebest defence an emplyee has to the question of why he did not compain when he was working there would be "I was afraid to loose my job." Just having that feeling is enough.
Once case I know of is where a manager who signed that he would do extra hours for no extra pay documented all the extra hours and got them payed out, because that is what the law says. Say they had to pay several years of extra hours.
Now before you start suing your company, please go to a lawyer (or your union) IRL, because every situation is different.