If you are using Turck MMCache and, in IMP:
Here is an example image of the issue:
(Also see http://marc.info/?l=imp&m=108163709627707&w=2 for example symptoms)
Go here for instructions on how to fix your problem:
Make sure the cache is deleted everytime Apache is restarted:
After some testing I entered the following to my php.ini:
Turck MMCache seems to be dead. There's a fork called eAccelerator (http://eaccelerator.sourceforge.net/). This problem should be fixed in the CVS version, but it is not yet fixed in version 0.9.3.