Basic Trope: A character manages to hide his identity with a disguise that shouldn't have fooled anyone.
Straight: Bob wants to spy on Dan, the Corrupt Corporate Executive. Although they know each other well, once Bob puts on a pair of sunglasses and a suit, Dan doesn't recognize him and tells Bob his plan.
Dan doesn't recognize Bob because he's carrying a briefcase.
Bob puts on his sunglasses and suit and nobody recognizes him.
Dan can identify everyone else on the planet (including the people he never met) even if they had put on elaborate disguises. Of course, the only one he can't identify is Bob, even without a disguise himself.
Bob messes up his hair a bit. No one recognizes him.
Dan is somewhat suspicious of the disguise, but ultimately decides he's being paranoid.
Bob is sometimes identified, sometimes not.
Dan is blind, so it makes sense that Bob disguising his voice is enough.
Dan has only met Bob in person once.
Dan is The Ditz. Anyone else would see through the disguise, but Dan is just too stupid.
Bob is a very good actor, so a simple change in clothing, accompanied by a change in mannerisms, can make him seem like a totally different person.
Bob's literally paper-thin disguise consists of cutting the eyes and nose out of a picture of somebody else's face and taping it onto his own. Bonus points if some combination of the following applies: the subject of the original picture is famous, has a distinctly different skin tone from Bob, the picture is clearly drawn, or in black and white.
Deconstructed: Bob comes from an extremely superficial society in which people recognize one another not by any fundamental traits but by surface physical appearance. The author uses the success of Bob's ridiculous "disguise" to comment on human superficiality.
Reconstructed: If someone were to figure out his identity, or Bob were to tell someone his identity, it will inspire a new wave of free thought and creativity.
Played For Laughs: Bob puts on a pair of bunny ears. Dan mistakes him for a rabbit and ignores him.