I encountered several times this week a critical incident alert in Enterprise Manager as generic internal error notifying for Internal error (ORA-600[qsmmixInitKey-2]) detected in alert\log.xml file.
After investigation it turned out that the reason can be in SQL Tuning Advisor.
The database is 22.214.171.124 . I have Automatic Maintenance Tasks configured and the Automatic SQL Tuning is also enabled.
You can refer to Metalink [ID 9602359.8] that gives overview of this Bug 9602359 OERI[qsmmixInitKey-2] using SQL Tuning Advisor.
Version confirmed as being affected is 126.96.36.199 and 188.8.131.52 for most platforms. In my case the production database is 184.108.40.206. The symptom is occurring internal ORA-600 [qsmmixInitKey-2]. Metalink says that this error can occur when SQL being tuned has duplicate columns. This issue is fixed in 220.127.116.11(Server Patch Set).
In my case I temporary disabled Automatic SQL Tuning and start planning the upgrade to 18.104.22.168.
dbms_auto_task_admin.disable(client_name => 'sql tuning advisor', operation => NULL, window_name => NULL);
This week I had one successful upgrade from 22.214.171.124 to 126.96.36.199 for Database and Grid Infrastructure on RHEL5.
This time I will plan and do the upgrade but on Windows Platform.