$ oerr ora 604
00604, 00000, "error occurred at recursive SQL level %s"
// *Cause:  An error occurred while processing a recursive SQL statement
//         (a statement applying to internal dictionary tables).
// *Action: If the situation described in the next error on the stack
//         can be corrected, do so; otherwise contact Oracle Support.

$ oerr ora 18
00018, 00000, "maximum number of sessions exceeded"
// *Cause:  All session state objects are in use.
// *Action: Increase the value of the SESSIONS initialization parameter.

$ oerr ora 6512
06512, 00000, "at %sline %s"
// *Cause:   Backtrace message as the stack is unwound by unhandled
//           exceptions.
// *Action:  Fix the problem causing the exception or write an exception
//           handler for this condition. Or you may need to contact your
//           application administrator or DBA.