In the one-shot Hiding in Plain Sight Harry Potter gets Lasik eye surgery and becomes completely unrecognizable to the pureblood wizards at Hogwarts. Then the Aurors try to recreate 21 Jump Street at Hogwarts. Again, the muggleborns have absolutely no trouble recognizing the incredibly out-of-place "first-year students".
A large degree of Kirby fanfiction gives Kirby the ability to become completely indistinguishable from whoever he's taken the power/costume of, despite the obvious size differences.
In the Calvin At Camp episode "The New Kid," Larry Koopa infiltrates the camp while wearing nothing but a Mickey Mouse hat.
In Nobody Dies, in order to pacify a rampaging A.I. based on Fallout 3, Gendo tricks it into thinking he's Abraham Lincoln simply by wearing a top hat. It probably helped that he already had the beard.
Hivefled: in one of the less grimdark scenes, the four trolls with the smallest and thus most easily-concealed horns dress up as humans to go food shopping. Not only are they wearing hats and gloves in June and concealing their grey skin only with face-paint, but John only just manages to stop them talking about eating babies in public.