Hello! I didn't see this documented anywhere, though I might have simply looked in the wrong places. In Generation VI (confirmed on the latest versions of Alpha Sapphire and X, anyway), if you use a multi-turn move such as Fly, Dive, Shadow Force, Dig, etc. and that Pokemon somehow gains enough experience to level up, the previous level is displayed until the game has an opportunity to "refresh" that area. I demonstrate this by using a level one Fletchling in a Double Battle.
Unfortunately, it's just a minor visual error. No negative HP or arbitrary code execution or something fancy like that. I couldn't think of any potential spinoffs that would lead to something, but if someone thinks of anything, I'd love to hear about it.