Since Oracle RDBMS 11g it is possible to use block change tracking on a standby database.
After using a snapshot standby database we got instnace crashes with:
ORA-00600 [krccckp_scn_low] - from CTWR Process
not exactly shure if this is a bug or missing dokumentation, but it seems that standby block change tracking is not handled correctly.
so if a 12.1 snapshot standby database is reverted to a physical standby, I recomment do disable and enable block change traking.
So this ORA-00600 [krccckp_scn_low] can be avoided.