If you have WordPress installed you may run in to an error that constantly prints something like this:
Warning: Unknown: Releasing SysV semaphore id 58 key 0x152b in request cleanup in Unknown on line 0
This is not an error from WordPress, but from the WP Super Cache Plugin you forgot you installed. The error comes when from the cache directory is not writable.
Change the permissions on the wp-content/cache directory to allow the web user to write and the error will disappear.
The solution is obvious if you check the source code on the page when the error appears. WP Super Cache spits out the error, but it is hidden in HTML comment tags. Your source code will look something like this:
<!-- Dynamic Page Served (once) in 0.297 seconds -->
<!-- File not cached! Super Cache Couldn't write to: wp-content/cache/wp-cache-2876abde8ce3a2aa817c8289ea85486e.html -->
<b>Warning</b>: Unknown: Releasing SysV semaphore id 58 key 0x152b in request cleanup in <b>Unknown</b> on line <b>0</b><br />