Thu Jun 09 11:56:56 2011
Errors in file /u01/product/oracle/diag/rdbms/ccptdb/ccptdb1/trace/ccptdb1_mmon_15531.trc  (incident=256273):
ORA-00600: internal error code, arguments: [kebm_mmon_main_1], [28], [], [], [], [], [], [], [], [], [], []
ORA-00028: your session has been killed
Incident details in: /u01/product/oracle/diag/rdbms/ccptdb/ccptdb1/incident/incdir_256273/ccptdb1_mmon_15531_i256273.trc

 
Cause

This is due to unpublished bug:8642529 fixed in 12.1 and 11.2.0.2, where the ORA-600 [kebm_mmon_main_1] error was raised if MMON encountered a user error.