Something I've been wrecking my brain with for a bit today, why do you make map files mirrored for games? The only thing I can come up with is that you would scan the file before you use it to look for errors in it and allocate sufficient RAM (hence avoiding hardcoded limits). Reading it from the back would be faster after that then reading it forward again as you don't have to worry about limits (you just look for the beginning of the file) and you don't have to move the filepointer to the top again.
But that doesn't explain why for instance sprite files are not mirrored, so while I'm rather sure the above thing probably would save some time (not all that much though) it doesn't quite explain why the files are mirrored, anyone know? :)