Some daemons can pop up with the following error (apache is one of them)

 semget: No space left on device

This doesn' have anything to do with a full disk, but the system cannot allocate any more semaphores. This error tends to pop up if apache is stopped with kill -9 too much, it won't free the semaphores then. To free them up become root and issue the following command:

 $ su -
 # for ID in `ipcs -s | grep '<user>' | awk '{print $2}'`; do ipcrm -s $ID; done

<user> must be replaced by the user under which apache runs, when it runs as httpd for example:

 $ su -
 # for ID in `ipcs -s | grep 'httpd' | awk '{print $2}'`; do ipcrm -s $ID; done

What happens:

ipcs -s  : lists all allocated semaphores
grep '<user>'  : only shows that user
awk '{print $2}' : Cuts the list and only leaves the semaphore ID
ipcrm -s $ID  : Deletes the semaphore ID