So this is a minor glitch I've seen several people bring up on /r/Pokemon, but there hasn't been too much research into it. There is one video showing the bug, which is actually how I first learned of it. https://www.youtube.com/watch?v=XgV8nrZceK4
It appears that this glitch is replicatable as several other people have reported encountering it.
I haven't been able to test this glitch myself due to only having one copy of the game, but if someone would want to test it over Wifi battle I'd be more than happy.
I'll list the steps of this bug even though it's very easy, I may not be 100% correct on what you must do.
1. Your Pokemon should be a Mimikyu with an intact disguise, and your opponent needs a Pokemon with Sky Drop.
2. It appears that battle animations need to be off, but this needs to be tested.
3. Your opponent uses Sky Drop, and when it drops Mimikyu its disguse needs to break.
4. Your Mimikyu will be invisible!
Note: In the video the Mimikyu is holding a Red Card, but I am not sure if this factors into the glitch.
1. Do battle animations need to be off?
2. Does your opponent see (or I guess in this case, doesn't see) anything?
3. Can this be done with any other Pokemon?
4. Does switching out cause things to return to normal?
5. Is the Red Card needed?
One thing I do know is that between the Gen 5 Sky Drop glitch and this is that Sky Drop is one glitchy move!