WordPress Error: require_once [function.fopen]: failed to open stream: Permission denied

Recently encountered this error:

1

require_once [function.fopen]: failed to open stream: Permission denied

Searching the web resulted in finding many sites experiencing the same issue:

I checked the permissions on the file in question (in my case it was the header.php) and for some reason the file permissions had changed to 200. I changed them back to 644. If anyone has any idea why the permissions would have changed please let me know.

For more details about file permissions with WordPress please see:

https://codex.wordpress.org/Changing_File_Permissions