Viewer Myopia

So the hero can't see that his Unlucky Childhood Friend is in love with him, despite it being utterly obvious with all the longing glances she casts at his back. And the hero's Love Interest can't put together the blatant clues that her nebbishy partner and the masked vigilante are the same guy, even though her partner always disappears at times it would be good to run away, and the hero always shows up at the scene of crimes. And the hero didn't even see the Evil Plan the villain set up coming even though it's been so done to death that it should have been plain from a mile off.

What is wrong with these characters?! It's like they're not even watching the series they're in!

Viewer Myopia or Audience Awareness Advantage happens when the audience has a tendency to forget that the characters aren't being presented with the same information, view, and experience with the genre that the audience has. Things that are so utterly embedded in the public consciousness that everyone is assumed to know them wind up being applied to the characters as well, since obviously they're people too. Things the viewer takes for granted because they've been presented to them in an obvious way seem like they should be obvious to the character too, even if there's no real reason to think this. In addition, things the audience sees coming plainly because they're genre tropes we've been exposed to all our lives probably aren't quite so obvious to characters that have no clue they're living in a comic book or crime drama or action film.

This can get especially bad with series that have run long enough that even those who have never so much as read a single issue or watched a single episode can be assumed to have a reasonable chance of knowing these things. Probably not helping are a number of comedies and unthoughtful parodies that Lampshade this or take it to extremes, which can make the viewer feel like they're in on some joke the characters aren't. Making everyone and their dog aware of every single trope out there almost certainly doesn't help. ... Ahem.

Basically, if someone is saying "Oh come on, he was so obviously the Evil Overlord shapeshifted into a teenager and pretending to be your best friend for ten years all along!", they're indulging in this trope.

Anime and Manga

 * Hinata being in love with Naruto. Note that she tends to do her fawning over him from a distance, behind his back, or from around corners, often with him never even knowing she's there. But could it be more obvious?

Comics

 * Superman is probably the ultimate example of this, and has been for decades (since the Silver Age at the least). For instance, a remarkable number of criminals seemed to not only know that Superman was weakened by Kryptonite, but exactly what each color of Kryptonite would do... even if it was that color of Kryptonite's first time showing up. Because everyone knows Superman is hurt by Kryptonite!
 * But even more than that is the whole "people don't know Clark Kent is Superman wearing glasses" deal. Again, having gotten blatant enough that even characters in-universe who are in on the secret have mocked others for not getting it. And the number of times "Clark Kent and Superman are never in the same place together!" is used as evidence is almost appalling.

Film

 * Any time some Dumb Blonde in a horror movie is counted as being a complete idiot for going upstairs because the psycho killer is up there. I mean, we just saw him sneak up there, geez! And who goes upstairs in their own home, anyway? This also applies to Death by Sex and various other horror movie tropes where characters are referred to as "just asking to be killed" by going skinny-dipping and such. While stripping down in the woods or at the lake for a bit of naughty fun might not be the brightest thing ever, "Chainsaw-masked psycho killers" are not generally a warning given in sex ed class or a hazard tipped off by common sense, unless you know you're in a horror movie. The Scream movies (and their Scary Movie parodies) are thus essentially Viewer Myopia: The Movie.

Theatre

 * People with autism are more likely to have what psychologists call a "theory of mind deficit" or "mind blindness". The short play The Sally Anne Test by Simon Baron-Cohen (the cousin of a famous comic actor) invokes this as part of a psychological test designed to measure this in viewers who may have a learning disability. First, Sally and Anne are introduced, and the examiner makes sure viewers know their names. Sally is carrying a basket. Sally puts a marble in her basket, puts the basket down, and exits. Anne takes the marble out of Sally's basket and puts it in her own box. Sally returns, and viewers are asked where Sally is about to look for the marble. The viewer passes if he believes that Sally will first look in her own basket. A viewer who believes that Sally will first look in Anne's box has mind blindness and has fallen victim to this trope. Baron-Cohen originally staged it as a puppet show; Leslie and Frith staged a production with live actors to rule out the objection that children might not attribute theory of mind to dolls.