In the book Secret Service Chief, a former head of the US Secret Service tells about his investigating a gang that passed fake checks. They entered a store, cashed some checks, went outside, switched hats and jackets and went back in and cashed more checks with different names. Several times. To the same clerk.
This is actually very understandable, and surprisingly easy to pull off. It's called Change Blindness. Here's a video showing a 'magic trick' where the back of the cards change colorand so does the table, the background, and the shirts of both participants.
You know when the cashier just doesn't seem to care? Change blindness is a more technical word used in that sort of thing.
An author who interviewed Marilyn Monroe later wrote of an incident that occurred when they were walking down the street talking. The author was confused that, although they were in plain sight, no one seemed to recognize her. Monroe then said, "Do you want to see her?" She changed her posture, walk and way she was speaking to what she used in the movies and suddenly people saw Marilyn Monroe, movie star and sex symbol, and reacted accordingly.
A reporter witnessed Mel Gibson do something similar when accompanying him to the DMV. Mel visibly "turned off the charm", changed his expression and posture and put on a baseball cap. He made himself so inconspicuous that even the clerk who saw all his documents and renewed his license took no notice of the resemblance to a famous man named "Mel Gibson".
This article has a very funny real life example of this. A bald, bearded reporter wearing glasses showed up at NBC asking executives about Jay Leno's future... the funny thing is that they didn't know it was actually Jay Leno in disguise.
Justified in the case of a man who changed clothes to get Dr Karl to autograph different copies of the same book; Dr Karl suffers from a natural inability to recognise faces.
British TV prankster Jeremy Beadle was short, fat and bearded with one hand noticeably larger than the other, yet he frequently tricked people by wearing a false beard and dark glasses.
Tohru Furuya, in working on Mobile Suit Gundam 00, only lists his name as the narrator. He uses the pseudonym 'Noboru Sougetsu' as the voice of Ribbons Almark. However, fans' ears cannot be lied at and judging on how similar Ribbons and the narrator sounds like... The cover is blown easily, but Furuya insists on using the pseudonym. 'To differentiate between Ribbons and Amuro...' Yeah right...
There was never really a cover to begin with; Furuya and Bandai both were up-front about his being in 00. Also, Furuya said that the main reason he used the pseudonym was to keep from stealing attention away from the show's real stars.
Similarly, when Wendee Lee was working on Rurouni Kenshin, she used her actual name when playing Yumi Komagata, but using the pseudonym Elyse Floyd when playing Yahiko Myojin. Given that Yahiko's one of the main characters of the show, it doesn't take that long to recognize Lee's voice if you're familiar with her work.
The same thing happens with actor-turned-voice-actor Johnny Yong Bosch in Gate Keepers. He voices both the main protagonist and the eventual main antagonist. Only in the antagonist's role is he credited by his actual name. His role as the protagonist uses the alias Jim Taggert. Indeed, many anime dub VAs have various aliases that are employed for personal or contractual reasons, such as David Lucas (real name: Steven Jay Blum), Dorothy Meneldrez (real name: Dorothy Elias-Fahn), Heather Lee Joelson (real name: Melissa Fahn), and Jeremiah Freedman (real name: Joshua Seth). All of them (as well as Wendee Lee above) are well-known voices in the dub community, so it usually doesn't take long to figure out that a given name matches another.
Shakira, a famous singer, managed to spend an entire summer at UCLA posing as a normal person. She went by her middle name and dressed up in a cap and pants. The fact that it hit the news after she was done with the classes proved how effective her disguise was.
For an "undercover" story, a reporter dressed as a typical college student hung around a college's dining hall. However, he was immediately discovered, because the student body population was so small that everyone knew each other and immediately recognized that the reporter was not one of their classmates.
Sam Walton, founder of Wal-Mart Stores, Inc., was able to successfully sneak into his competitors' stores unnoticed by simply leaving his trademark hat in the car. Even Walton himself never was able to figure out why this was so effective.
Probably because most people have only a passing familiarity with who Sam Walton is and absolutely no idea what he looks like. Even if you do, he was basically the "old white guy with neat haircut" version of a The Nondescript in any case.
Some people have a mental condition called prosopagnosia that makes it difficult, if not outright impossible, to remember faces. So even a Paper-Thin Disguise can be enough to fool people with the condition.