Studies on attention that the human brain is capable of remarkable focus when given a task, to the exclusion of all else. For example, when asked to count the number of passing trucks in a video clip, many viewers will fail to notice the random guy dancing around in a gorilla suit (other variants involve a video camera focused on a couple of guys passing a basketball, with you being told to count how many passes they make). And despite spending significant time looking at such sights, participants in a driving study could not remember details of a scene or signage passed by.
Also recently, a researcher placed a matchbook-sized image of a gorilla in an x-ray that may or may not have barely detectable signs of a cancer. A surprisingly small percentage of radiologists noted the gorilla.
Channel 4 apparently messed this one up when they televised one of Derren Brown's live shows—he had a man in a gorilla suit come on stage and take a banana from a podium that the audience had already been told to watch out for, but at the time it happened they'd been told to concentrate on a woman's reactions shown on screen. Unfortunately for anyone watching on TV, the camera was also focused on the woman, meaning we really couldn't see the gorilla and don't much appreciate being mocked for missing it.
This is especially true for people with ADHD.
People also tend to notice, but not react because they don't want to get involved or interrupt whatever errand they are on unless there is a threat imminently endangering themselves or others.
A book about the making of Doctor Who tells about how, before the first Cyberman serial aired, the makers had someone walking around in the middle of London in a Cyberman costume for some reason. No one freaked out, no one asked what was going on, and there were almost no one reacted in any noticeable way. Best comment was from a fruits and veggies seller who said to his wife, "Must be an advert for one of them new kitchen cleaners."
The same thing happened prior to the filming of Star Trek IV. Paramount had people walking around San Francisco in full Starfleet uniform, or dressed as Vulcans, prior to location shooting. No one noticed.
This series of images with Bowser doing political protests.
"Open carry" is the law in several US states, meaning that unconcealed firearms are legal in public. Notably averted by media coverage of protesters outside a presidential event. A Secret Service spokesman noted that the individuals would never have gotten in close proximity to the president, regardless of any state laws on openly carrying weapons. A venue is considered a federal site when the Secret Service is protecting the president and weapons are not allowed on a federal site, he added.
Note, however, that many people do take notice if you walk around with a rifle on your back, and you will most likely be talked to by the police about it, unless you do it regularly. Or have other hunting gear equipped.
This guy walked into a JC Penney's at a shopping mall with a civilian semi automatic rifle slung on his shoulder and a pistol on his hip. Both perfectly legal, of course. There was less running and screaming than you might expect, particularly after these events.
Truth in Television: In big cities, you get a lot of strange people to the point where it becomes regular, and many people simply ignore odd behavior to avoid drama, and to not cause a fuss or offend someone for being different. It isn't so much that they don't notice things like strangely-dressed people as that they think it's not for them to deal with.
Also weird is relative, the natives of major con sites tend to not really notice cosplayers after they get use to seeing them. To them it's no different than a man dressed as Santa in the mall at Christmas. Of course, around con times, the amount of weirdness tolerated rises far beyond normal levels, to the point where one can walk around in full combat gear without being questioned.
Inverted in a common symptom in people affected by ADHD: Even things that are actually uninteresting are constantly getting a second look, making it difficult to stay focused on one object.
This all means that in New York City, there's probably a lot of outrageous stuff you can do without anyone so much as batting an eye at you.
Southern California, especially Hollywood's Sunset Strip and San Francisco, has people doing a ton of strange things, but no one really notices because it's all "doing their own thing". In the Castro, there are people who walk around completely naked as a way to make a statement about nudity laws, but they've been at it so long no one takes a second look.
College towns. There's nothing at all unusual about seeing a bunch of people outside up past midnight in the middle of January wearing who knows what.
Colleges themselves are full of this.
With the way science and technology has progressed over the last 300 years, this trope has repeated itself time and again as people are at first amazed when they see new sights like cars, mobile phones or TV sets but then come to accept them as normal.
There are (or were) some pictures on Facebook of somebody dressed as a Silent in a shopping centre, that no one noticed. note At least judging from the lack of tally marks.
Shuler Hensley, who played Frankenstein's Monster in Van Helsing, said he practiced using the stilts of his costume in Central Park and nobody even looked at him.
Halloween. People dressed in strange costumes that would look suspicious any other time of the year. Some people look like they are covered in blood, but everyone can tell it's fake. Averted for Devil's Night, the day before Halloween, because of historical violence on that day.
Pretty much any time anything unusual happens on the Tube (the one in London). Comedian Chris Ramsey (who's from Newcastle) once said that someone could literally burst into flames on the Tube and the most recognition they'd get would be some guy looking up from his Kindle and savagely muttering, "What a prick."
This photograph. A bikini-clad female Israeli soldier is at the beach with an assault rifle slung across her back and no one else at the beach seems to be paying attention to her. People in other countries were somewhat shocked to see this, but Israelis are used to seeing armed soldiers in many different places.