Surprises happen when you go to the latest version of Oracle without deep testing.
My client just hit the bug:
Bug 20513930 : UPDATE FAILS WITH [QERHJDISABLEHJCBK: NOT A HASH JOIN]
Basically the client was getting a ORA-600 [QERHJDISABLEHJCBK: NOT A HASH JOIN] when doing a certain query. Looking at it, it was an UPDATE of a view. This view had a “instead of update” trigger, which was doing several actions. After enabling trace on the session, the bug does not happen!
Fortunately someone had already open the bug at Metalink. The workaround is to disable the new “groundbreaking feature” (dixit Oracle) called Adaptive Plans.
SQL> alter system set "_optimizer_adaptive_plans"=false; System altered.
Hopefully there will be soon a patch for this problem.