It's crazy when googling D750 light leak.
It's entirely possible that dude's cam has an issue but that doesn't necessarily imply the D750 has an issue, just that his has. It's also possible he's a moron that dropped his cam down the stairs. If you see some reactions out there, it apparently doesn't take much to create a "D750 failure" hype. I wonder how many of them own one.
If it is a serious issue we'll hear that pretty soon but me thinks that an issue like this would have popped up much earlier, if not directly after release.