I've not seen this little thing posted ANYWHERE before, who knows, hell, maybe I discovered something. (Waits for someone to post a link and prove him wrong.)
Anyway, by loading the S3&K Level Select savestates on Sonic 2 & Knuckles, I've caused a glitch that causes Tails' sprite to be glitched and become all scrambled and whatnot, and jokingly, I've dubbed it "The MissingTails Glitch", as Tails bares some small resemblance to MissingNo in a way (In a small way, so don't get on me about it.)
MissingTails standing.
MissingTails jumping.
MissingTails flying.EDIT: A couple more shots of MissingTails:
MissingTails at full speed.
MissingTails spindashing.
MissingTails charging a spindash.EDIT 2: More MissingTails shots.
MissingTails taking damage.
MissingTails hanging.
MissingTails victory animation.EDIT 3: Also, here's a savestate that is for Sonic 2 & Knuckles, it places you in Mushroom Hill as Sonic, with MissingTails following behind.
(No, you cannot play as MissingTails alone, any attempt causes the game to stick at the selected level's title card.)