Nothing. Maybe. The magic happens in open() or createfile() (or a little underneath there, precisely). Nothing about the disk format prohibits such names, and they're accessible via other pathnames. But if you have a program that lists files and then tries to open each one automatically, it may have a bad time. I would hope AV software has tested this scenario, but AV software.
This is the type of question that leads to new classes of exploit... well thought!
Except it's one of the first things each of the thousands (likely more) of professional and hobbyist hackers have already tried after learning about this particular OS quirk