I have seen this on AIX, Solaris, Linux and others. Mostly when one of the DBAs has performed a shutdown abort or the system is shut down without the database being shut down first. In earlier versions of oracle (6&7) you would still see the sgadef file in OH/dbs. The startup sees this file (I believe a pointer into the shared memory segment) and believes the database is already started.
If the shared memory segment and semaphores are still there executing sqlplus will attach to the segment but you can't do anything because the background processes aren't running.
IMHO - if the processes get killed the segment and semaphores don't get released and the lock file is not removed.
Mike -- //www.freelists.org/webpage/oracle-l
