Some users report that they receive the following error when attempting to create new content (of any type):
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.
or with maximum reporting on
Fatal error: Invalid opcode 153/1/8. in /var/.../libraries/joomla/table/observer/tags.php on line 159
This appears to be caused by a bug in eAccelerator. Either update to the current version or change your caching method to a different option.