FTF = Failure to feed; when a round fails to feed from the magazine and seat itself properly in the gun's chamber. It's generally caused by a bad feed ramp, shit magazine, bad ammunition, or holding the firearm improperly.
He could also experience a stovepipe, which happens when the firearm fails to eject the spent casing and it gets stuck in the ejection port. This isn't at all uncommon with firearms, and can be caused by, again, holding the firearm improperly, a faulty extractor, or bad luck.
Turning the gun upside-down could cause both of these problems, thanks to gravity and poor recoil management. With most firearms, the hands have to manage the recoil of the firearm's frame while the slide moves back and a new round is chambered. If you aren't holding the firearm properly, there's a good chance the slide won't have enough momentum to properly cycle the action on its own, so you'll get a malfunction and look like a dick.
It's not really a design flaw, more like a, "You're not supposed to do that shit, because windmills(guns) do not work that way." thing.
It might seem a little complicated and tl;dr, but essentially, firearms are intended to be held firmly, with the sights facing up(unlike the "gangsta style", where the firearm is turned on its side). Lifted this pic of a group of Marine Security Guards firing their M9 service pistols off Failpedia for reference.