Many time during coding we need to same a copy of the code file. A quick backup of the work before a some may-go-wrong kind of situations. Do not just do a copy-paste of the file. In many systems like under CentOS the file gets saved as “MyController (copy).php”.
This can lead to nasty situation. While you might be thinking it that the actual file “MyController.php” was being used by Typo3 it could very well be otherwise .i.e. Typo3 might just be using the copy “MyController (copy).php”!
So be safe and always add a prefix to the backup file name.