Operation Not Permitted

This appears to be regularly confused with EACCES. To quote from SUSv3:

Operation not permitted. An attempt was made to perform an operation limited to processes with appropriate privileges or to the owner of a file or other resource.

Examples include:

• Trying to unlink(2) a file on a read-only filesystem.
• Trying to increase a process's priority by giving the nice(2) call a negative value when not the superuser (note that this is different to the setpriority(2) call!)
• Trying to use the setpriority(2) call on a process that does not belong to you.

In general, EACCES seems morely to be used for file permission conflicts, while EPERM seems more likely to be used for process-related permission conflicts. But you can see that even system calls use them differently.