Is it possible and are there cases where patents have been granted even though there is prior art that describes the same invention? Do these patent grants have to be reverted when someone finds prior art describing the same invention after the patent opposition period?
There are different procedures in various jurisdictions. In no place I have heard it is automatic or easy. Until the 2012 AIA law in the US patents could only be invalidated in a court process that involved an alleged infringement.
Now there is a way for any party to challenge the validity of an issued patent based on prior art through an administrative, court-like process. It is still relatively expensive.
In Europe, usually each lawsuit for patent infringement against a third party is responded to with an invalidity counterclaim. It is fairly common to see patents being revoked in Court.
In the case of European patents, once the patent has issued, third parties are allowed to file an opposition with the European Patent Office in an attempt to limit the scope of the patent or revoke it entirely. Unlike in the US, the cost of opposition before the EPO is relatively low. The filing of oppositions is not so uncommon. See for example the EPO's calendar of oral proceedings, each day there are 10 to 20 oppositions in average.