It's implied that Equius's "Heir of Void" power amounts to being able to make things impossible to spot check. This can range from pixelization on musclebeast penises to hiding something from the omniscient view of a god. Specifically, it was implied with his Ancestor Darkleer, who had the ability of the Void.
Previous to this, Dave Strider manages to mistake the apocalypse for a heatwave.
On a more meta level, it took almost two years for the fans to notice that Dave's hair is a bird, which subsequently became a meme.
In Knights of Buena Vista, Mary races her PC down a street to foil a robbery, but rolls a 2 to see if she notices Bill's character riding right to her. That's how Anna in Frozen got knocked into the boat.
The Looking for Group crew does this once. Somehow they fail to notice the dragon perched conspicuously atop the spire in the middle of the cavern while they go down into the crater to play with the eggs. Oops. Apparently, it was a statue before they touched the eggs.
In MegaTokyo, countless photos of Miho are uploaded online by Ping, causing her to go from zero online presence to massive meme overnight. Piro assumed it was a jilted ex-boyfriend, even though Ping stamped every individual photo with her serial number.
Piro: Huh? Ping? Miho: Who else would it be?? These are all from when she was with me!! Piro: How was I supposed to know that? Miho: You can even see her in the mirror!! Piro: What? I never saw Ping— Miho:That's because you were too busy staring at my underwear!!
Mulder and Scully of Monster of the Week repeatedly fail a spot check, and (surprisingly) it's rarely lampshaded. For example, Mulder doesn't notice that the Senator who ordered him to investigate the aliens has a photo of himself shaking an alien's hand, and Scully doesn't notice that her doctor's surname is Evil.
Yeagar doesn't seem to notice that the guy he's asking for directions is a demon from the pits of Hell (albeit a polite one) carrying a victim in one strip of Nodwick.
Belkar: Wait! I think I just failed a Listen check!
By the rules, ninja are hard to spot (and automatically considered hiding regardless of circumstances). Belkar can't even spot the ninja panel in the bonus page about the Pirates vs. Ninjas controversy.
In Sleepless Domain Heartful Punch somehow missed that three members of Team Alchemical died, despite it being city-wide headlines and there being a school assembly on it (which she did admit she skipped). When she comes across Undine alone, she just thought they broke up, and Undine doesn't disabuse her of that notion because she appreciated being treated normally. HP feels horrible about how insensitive she inadvertently was when she finds out what happened.
In "Art Belial", Aylee calls Riff for help because she can't find her way out of Art Belal's (sic)note Yes, it's spelled differently in the story name large compound. When he gets there and the creatures living inside agree to help them find the exit, they point to a big obvious door with the word "EXIT" all over it.
In "Most Wonderful Time", someone has stolen all the main characters' Christmas stuff, including the tree. Eventually Riff and Zoë notice Kiki's hidden it under the couch they're sitting on wondering where it's all gone. The tree doesn't remotely fit under the couch and they haven't been noticing the fact that the couch is at about a thirty-degree angle from the ground. To the artist's credit, the reader probably won't notice the angle either before the view zooms out.
In "Years of Yarncraft", when Riff plays an MMORPG, he's frustrated trying to get an item that's only dropped by female slimeblobs, which he can't tell apart from the males. It takes Torg to point out that, like all women in the game, the female slimeblobs all have the bodies of supermodels.
Riff: In retrospect, why the hell didn't I notice that sooner?
In "Homeward", Riff invents a joy buzzer version of his overpowered Omnitaser Supreme, but it's a relatively large contraption and way too visible for anyone to miss it — until he paints it camo, which makes it no less visible but still, illogically, works.
Subverted in "The Research and Development Wars": Torg tells a story about the architect who designed the building they're now in and, due to having made its core into some kind of an inescapable Möbius strip shape was trapped there forever. Zoë asks where he is now if that's the case. Torg points to a skeleton sitting right next to her. She states that she "totally thought he was part of [their] team" — which while she was away had come to actually include a vampire and a zombie head on a stick.
Grymm and Creepknight from Voodoo Walrus make a bad habit of this when it comes to not noticing that they're being tailed by ninjas or various henchmen looking to ruin their day. Everyone but Mirth seem to be susceptible to this when it comes to the strange spooky yet cute creatures that regularly pop up in places without any explanation at all.