ALERT: DB corruption after 12c upgrade for compressed tables during recovery

Did you compress tables on 11g before going to 12c?  If so, you need to check out My Oracle Support article "ALERT Bug 21682261 Corruption during Recovery after upgrading to 12c for Compressed Tables" from document ID 2058461.1 because you can run into corrupted tables if you need to recover them!  This is the worst possible news for a database because it violates all of the ACID principles and if you get to the recovery stage you already have your data in question!