This internal error is because a background process is not correctly invoked hence terminated. That means the type of the detached process cannot be determined. In other words, this error can show up for a number of reasons. Bug 868241 indicates there were problems with this error message showing up if you exceeded the parallel_max_servers value--fixed in 8.0.6. Bug 728866 indicates this can also occur when spawned PQ slave processes fail to map to the SGA.
ORA-600: signals that an Oracle process was started but it cannot work out what it is supposed to be doing, so it exits with this error. It has been reported that simply typing 'oracle' at the command line can cause this error. It has also been reported that having log_archive_start=TRUE in the init.ora, but the database is not running in archivelog mode can cause this error. There was a bug in earlier versions where this could be signalled due to a timeout when starting parallel query slaves in an OPS environment, but it was fixed in 8.0.6.
I faced this problem twice:
i) wanted to take backup of datafiles of an NT/ora8.0 setup from a
remote machine into dat by setting the permission of this folder as
read only to all. then tried to start up the database when the backup
is complete. but did not change the permission.
ii) also faced the same ora-600 error due to data block corruption.