The ORA 600 errors are usually internal system errors - in the full error text there'll be some other numbers that may enable Oracle's support staff to identify a known problem in their helpdesk system if you're lucky; they can be quite serious i.e. you can't start the database, or spurious, showing up once during particularly heavy load and never again.<br>
If there are serious effects or you're getting lots of them note down the full error text and make that call to the helpdesk, or perhaps try the Oracle: Oracle 5/6/7 or Oracle: Oracle 8 forum as appropriate.