"Space Pilot 3000": Matt Groening's head is visible for exactly one frame in the Head Museum.
"A Head in the Polls": Bender has a nightmare where he sees "1s and 0s everywhere! And I think I just saw a 2!" If you pay attention, you can actually see said "2" during the aforementioned Dream Sequence.
"Jurassic Bark": In the museum, 'Coprolites (= fossilized animal feces) of the New York Knicks'
The season three finale of The Venture Bros. has a single frame nip-slip of Dr. Girlfriend.
In universe, the Rusty Venture Show had single frames hiding clues that lead to the location of the ORB, which Billy decoded after years of research and luck.
The Simpsons: In the intro, the fast camera pan from a Springfield street to the Simpsons' garage looks like a blur at regular speed, but if you slow it down, you can see that it's actually a legitimate (albeit warped) pan across a HECK of a lot of characters.
The new Simpsons intro includes a billboard during the camera pan of Springfield. It changes every episode, but is usually impossible to read without freeze-frame.
In "Bart's Comet", when the whole town crowds into Ned's bomb shelter, you can briefly see Waldo the first time everyone is shown crowded in.
In "Bart Star", in the scene where the Hills make a cameo, Bill, Dale, and Boomhauer can be seen in the background.
An in-universe example during the episode "I Am Furious (Yellow)" — when the cartoon "Danger Dog" is shown to the school, the creator freezes the frame showing that the chunks of barf are pictures of the animators' and their friends' heads.
If there's a sign for a public event in the Simpsons there's probably something hilarious and easily over looked, such as the "Semi-annual Halloween Festival" or the candy convention next to the candy shaped poison convention.
The Intro reveals that Maggie Simpson "scans" as $847.63 in the grocery store sequence, which was the monthly cost of raising an infant when the show premiered.
When Bart destroys Former President George Bush's memoirs in "Two Bad Neighbors", a small shred of paper briefly passes in front of the camera on which the words "V.P. Quayle" and "embarrassment" are legible.
The end-credits for the in-universe film, "The Poke of Zorro", in episode E-I-E-I(Annoyed Grunt) also qualify. Hilarious highlights include Posh Spice as "Wise Nun" with Meryl Streep as "Stupid Nun", Cheech Marin as King Arthur and James Earl Jones as "Voice of Magic Taco". There are also shout-outs to John Byner, Eric Roberts, Pele, and a personal favourite, Spalding Gray as "Gay Seeming Prince".
Near the end of the episode "Homer Badman" when Rock Bottom shows the list of all their corrections you can read some pretty humorous stuff such as "Licking an electrical outlet will not turn you into a Mighty Morphin Power Ranger.", "The Beatles haven't reunited to enter kick boxing competitions" and "if you are able to read this you have no life"
Near the end of the Justice League episode "Starcrossed, Part III", there's a single-frame Hit Flash when Hro Talek strikes Hawkgirl. This frame is not pure white, but a very washed-out closeup picture of Hawkgirl screaming.
In the JLU episode Grudge MatchNightwing makes a blink-and-you-miss-it cameo as Black Canary and Huntress enter Bludhaven.
The intro has this sentence briefly appear: "Don't tell Brain I hid this secret message. Ha ha Narf!"
As well as "I knew about your silly secret message, Pinky."
Freakazoid! does this a lot in its end credit. One episode (which features an extremely long musical number involving the arch-villain the Lobe) contains a rather lengthy (and humorous) justification for it that is not legible without freeze frame: "One time a space ship full of aliens came down and kidnapped our writing staff. The aliens threatened to blow up the Earth with a solonite bomb unless the writing staff wrote a big, splashy musical number for the Lobe. The writing staff worked for days on the song. The aliens were so pleased with it they agreed not to blow up the Earth. So the next time someone says "I don't know why they did that real long musical number in Freakazoid," you can tell them. You can tell them of the time a rag-tag group of cartoon writers saved the Earth from destruction, and that each new day that dawns over this great, big blue planet of ours is due to these brave individuals."
And of course, Emmett Nervend. At the end of each episode they give you a helpful guide to the total number of times he's appeared.
Twice in the Mickey Mouse cartoon Runaway Brain, Zazu the hornbill from The Lion King appears on screen for a few frames. The first time, he's among the debris being sucked into the trap door, and later he comes out of the monster's mouth as he roars at Mickey.
Invader Zim: Every five episodes or so, there is a single frame flash of a bloodsoaked GIR. He has something of a blank stare on his face and is in red-eyed "Duty mode." The image itself can be found here.◊
In "The Girl Who Cried Gnome", while the camera is panning down to Zim's base after the robot gopher is summoned for the first time, one can see that there is an apparently live puppy buried somewhere between the Earth's surface and Zim's underground lair.
In "Halloween Spectacular of Spooky Doom" for a couple of scenes a demonic Johnny of Johnny the Homicidal Maniac can be seen among the monsters.
One episode of Clone High features flashes just quick enough that you can tell they're going on throughout the episode. In one, Cleo's clothes vanish leaving her wearing only flower-shaped pasties; in another, the words "Scudworth is your favourite character" appear. In the finale episode, Joan of Arc is found naked in bed and quickly pulls the covers up over herself, but if you pause before she does so her left nipple is replaced with the word "NICE" and the right one with "TRY".
I BURIED PONCE
In the (John) Thomas Jefferson episode of Frankenhole, a scene in the White House's "Oval Office" features a rug with the moderately hard-to-read legend, "Was It Worth The TiVO Pause?".
In the Batman: The Animated Series episode "Two Face," it's easy to spot in normal viewing, but still cool to freeze frame Harvey Dent's gruesome half foreshadowed in a flash of lightning.
At the end of the episode "The Grey Ghost", look at the top of the blown up People magazine cover when they pan up to show it clearly, and you'll see "Matt Hagen: Man of a Thousand Faces" across the top above the People logo.
At "The Clock King" at Schedule Fanatic Temple Fugate’s screen laptop we see briefly:
Things to do today – 5/12
Not so easy to spot, but knowing what they did in the above example, you know to go back and look for it. In the Batman Beyond episode "Out of the Past," Just as Ra's Al Ghul in the body of Talia is about to run through Batman (Terry McGinnis) with a sword, he catches the blade in his hands and delivers the sword into the computer's control panel. This sends a highly visible electrical current but non-lethal electrocution enveloping Talia's body where in a couple of frames, Ra's Al Ghul, whose essence had completely usurped Talia's existence, is seen matching Talia's agony during the electrocution.
In the Superman: The Animated Series episode "Mxyzpixilated", Jimmy hands Clark a comic strip page featuring the titular Mr. Mxyzptlk. There are other comics on the page, Dini The Meany (parodying Calvin And Hobbes and Dennis the Menace and also referencing one of the show's producers, Paul Dini), Gleen (parodying Peanuts and referencing Glen Murakami), Dan Danger (parodying Dick Tracy and referencing Dan Riba) and Zub Street (parodying Momma). You cannot read these strips without freeze framing. Check out the screengrab!◊
As a bonus: the author of the Dini The Meany strip is called Bill Wemissu in a reference to Bill Watterson (Wemissu = "we miss you"), and the authors of Myxzptlk's strip are... Siegel and Shuster!
A less humorous one that's mentioned in the commentary is in the episode where Lex Luthor makes a deal with Brainiac. If you freeze during the bright flash when Brainiac's ship opens, you can see Krypton. The commentary implies that it was a Throw It In moment.
In the early Betty Boop cartoons some of them would contain a single frame of a nude Betty.
In the 90's Looney Tunes short "CarrotBlanca" when Daffy is carrying Bugs' luggage he trips and drops it and for a few frames Elmer Fudd can be seen falling from it.
In 1943's Porky Pig's Feat, Daffy Duck is challenging the hotel manager to a duel ("You have insult me! We meet on the field of onion!"). There is a frame where a cel of Daffy is photographed painted side up, which is viewable if it is frozen at the right spot.
In "The Dover Boys" when Dan Backslide says "confound those Dover Boys they drive me to drink!" and rapidly drinks down shots of whiskey, at one point if you pause it frame by frame you can see the bartender sneak one of the drinks for himself.
In "The Great Piggy Bank Robbery" at the beginning of Daffy's dream when in silhouette he says "I'm gonna pin it on ya see" his silhouette briefly turns into Dick Tracy's for a couple frames.
In an episode of Regular Show called "But I Have A Receipt," Mordecai and Rigby are spreading the word about how terrible a RPG The Realm Of Darthon is. One scene they show people all the classes of the game, and Mordecai proceeds to flip through the game book. It looks like a blur after the first couple pages, but if you have a quick pause button finger, you can see classes such as Scorpion Lawyer, Goat-Legged Dude, and Space Colon in the book.
In the episode "Starter Pack", if you are fast, you can read "Interns do it for free" in Thomas' bumper sticker.
Also in "Weekend at Benson's". You can notice the jar of Habanero Peppers that Benson eats from says "Pepper on Pepper action".
In the "solid" episode, the clock reads 9:00, but later 4:20 on the VCR.
In the first episode, when Twilight is talking about Nightmare Moon and the Hourglass passes over the unicorn head statue in the background, it changes to look like Nightmare Moon, going back when it is no longer behind the Hourglass.
In Griffon the Brush Off, when Rainbow Dash and Gilda race to a cloud, each claims to have been first, but Pinkie Pie claims Rainbow Dash won by just a little bit. To the naked eye it looks like a draw, with both arriving in a flash at the same time, but freezing the image at the right time shows Pinkie is right.
In Sonic Rainboom, when Twilight Sparkle casts the wing-making spell, an outline of Rarity with her butterfly wings appears for a couple of frames in the explosion of light, several minutes before we get to see the results of the spell properly.
When her friends dismiss the broken table as who wants a hammer for a cutie mark, Apple Bloom looks dreadfully upset.
Just before Sweetie Belle starts singing, Scootaloo and Apple Bloom cover their ears.
In The Cutie Mark Chronicles, during Fluttershy's flashback, at the end of her song, if you watch the upper-left corner of the screen carefully, you can see Rainbow Dash just as she hits her first Sonic Rainboom. See here.◊
In Sisterhooves Social, Sweetie Belle runs a race with AppleJack. At the very beginning AppleJack gets completely covered in mud and they run the race together, it's only until the end it's revealed that Rarity was really the one who ran the race. If you look back at "AppleJack's" eyes you'll see that they're blue just like Rarity's, AppleJack's eyes are green.
The dog anatomy pictures in the Vet's room all have the outline of Winona, Applejack's dog.
When Spike recovers his normal size and Rarity says "YOU were the rampaging dragon?" Spike grins and strikes a "let me explain" pose for half a second before plummeting to the ground.
In Baby Cakes, if you look closely during the opening, the babies in the nursery have the same or similar color schemes as other ponies, though most of them match more than one other pony (two that can be pinned down are the ones matching Rainbowshine and Medley).
In the stands near the royalty box is a pegasus mare with Bulk Biceps' body shape and the atypical beehive hairdo, butterfly glasses, and a string of pearls necklace. She has been dubbed "Mama Biceps".
The necklace Tirek wears in the present is the same one Scorpan is shown wearing during their backstory.
Discord-as-a-pony's cutie mark resembles a cyclone or tornado. Appropriate for the spirit of chaos.
In Twilights Kingdom Part 2, the cucumber sandwiches Discord offers Fluttershy are cut into the shape of butterflies, or Fluttershy's cutie mark, which makes it even more insulting when he drops them on the ground in front of her.
Derpy Hooves'cameos are sometimes this, like in "May the Best Pet Win", where she peeks out of Fluttershy's chicken coop near the end of the "Finding a Pet" song.
This promo from the Hub riffs on the 'There's an App for That' adds from Apple. Every pony app shown has an appropriately punny name and a matching icon (e.g. Wikiponia, Hooftube, etc.), proper use of pony terminology for various app functions, and even the 'PNN' news article with tiny text which flashes by at the end is fully coherent and readable. Someone at The Hub obviously knew that that the Periphery Demographic would examine every frame for details like that, going so far as to give a Shout-Out to popular Pony Blog Equestria Daily.
In South Park episode "Damien", when Damien makes things fly in the cafeteria, you can see Mr. Hankey flying across the screen.
During a thunderstorm in "Rainforest Shmainforest", Stan cries out "Oh my god! I just saw Tony Danza!" The tour guide doesn't believe him until an undistinguished, dark-haired adult male suddenly appears squatting next to the tour group as a flash of lightning briefly illuminates them.
When the six-graders amass at the Return of the fellowship of the two towers, Scott Tenorman can be seen; even though last season he was practically mentally destroyed in Scott Tenorman Must Die(And even created a Ginger-underground gang as a Vendetta against Cartman in a future episode, after going to treatment), but he now has the luxury of acting like a regular boy douche.
In a Sealab 2021 episode, Sparks recognizes a line Marco uses for inspiration. As he cycles through his library of movies, a good majority of them (read: all but one) were tentacle-rape hentai, all with hilarious titles and descriptions, but only shown for a second at most.
In Dexter's Laboratory during the episode "Dexter's Rival", during a scene with Dexter at his school desk, if you pause before the camera zooms in on Dexter there is a Bart Simpson-esque character at the desk to the left.
The Beatles cartoon episode "Strawberry Fields" has the boys being pummeled by rocks and mudballs by a gang of frightened orphan children at a run-down orphanage. During the pummeling, a zoom-in of a cartoon rendition of Private Gripweed—John Lennon's character in the movie How I Won The War—can be seen.
In the Home Movies episode "Curses", Brandon mentions he thought his censored movie was so boring he threw in brief frames of dogs having sex. If you go frame-by-frame around this moment, you will see one.
In The Problem Solverz episode "Funny Facez", many characters◊ from past and future episodes appear in the audience at the funny face show. There's Lidget, Dork Face, one of the alien Girl Scouts and Dr. Kunichi.
In the Adventure Time episode "The Real You", Finn puts on mind-enhancing magical glasses, and an Astronomic Zoom happens to establish his sudden understanding of the universe. When it zooms in on the planet, you may not think that there's anything odd about it. But if you pause just as it flashes by, you can see that the planet has an enormous crater in it. Like, a moon-sized crater.
A snail makes a brief appearance in the background roughly Once per Episode (complete list of appearances here). As of the season 2 finale, it has been possessed by The Lich.
In "What Is Life?", the pictures in the book Finn reads while brain-storming ways to prank Jake have various silly captions◊.
In "Mystery Train", while Finn is flipping through the pages of the dictionary, you can see a few entries for other words, including "mathematical", "Ice King", and "pancake" (one entry for which reads "Natasha's pet cat").
In "Hug Wolf", you can briefly see the book in which Jake looks up Hug Wolves has the title "Bestiarium Vocabulum", with the subtitle "(Beast Compendium)", and another subtitle that reads "((Animal Book))", including the extra parentheses.
In "Gotcha!", when Lumpy Space Princess is working on her book near the end of the episode, you can catch a glimpse of one page.
Pause at the right time in "Reign of Gunthers", and you'll see that the Ice King's web search history has some amusing entries, like "wants to rub mayonnaise on a penguin", "wiping my bum isn't working right", and "wrong way to eat a banana".
There's a split-second close-up of a newspaper article featuring Simon Petrikov, the Ice King's pre-Mushroom War self, describing how he found a ancient book. Said book is actually The Heroes' Enchiridion.
In "The Lich", during Finn's dream there is a blink-and-you-miss-it frame of the Lich laughing in Billy's place, as if Billy IS the Lich. If you catch it, it foreshadows a pretty big twist in the plot later in the episode.
In the Ren and Stimpy episode "Royal Canadian Kilted Yaksmen" when they give there victory cheer and throw their hats and kilts up in the air along with an intestine, right before it falls out of frame a turd wearing a kilt pops out of it.
In the Animaniacs episode "Garage Sale of the Century", in the scene where the neighborhood gangs up on the bear to get their money back, if you pause it frame by frame you can see Batman, Buster, Babs, and Dizzy from Tiny Toons, and the Marx Brothers among the crowd members.
The Thief and the Cobbler has a scene where the eponymous Thief is shot out of a cannon. If one freeze-frames the movie at the scene where the Thief is facing the camera, the flies perpetually circling his head turn out to be minature Thieves with wings, a testament to the animators' talent and determination.
In the King of the Hill episode "A Firefighting We Will Go" when Hank arrives at the fire station with a Frito pie and looks at the messy room, if you pause it you will see a drawing of a pig labeled "Bill" presumably done by Dale.
The Daria fandom is particularly liable to focus on minor background characters, giving them Fanon and making them into OC Stand Ins. This is aided by the episode "Café Disaffecto," where one can briefly see Mr. O'Neill's seating charts and thus get canonical names. For example, "Burnout Girl" is apparently named Jennifer, and thus commonly referred to as "Jennifer Burns" in fanfiction.
Several bonuses are hidden in Gravity Falls, including coded messages in the credits, and an image of Bigfoot in the show's theme song.
There's also a classified document that's shown on the screen — a biography of a character on the show. The characters who discover it glance over it briefly and then say the important information out loud, but if you pause (or read really quickly), you'll find that the document includes the paragraph: "“Let it be here recorded… Nathaniel Northwest, famous in his native Gravity Falls for standing in the park and hitting himself with a large boating oar until he blacked out, was chosen to be the patsy mayor of Gravity Falls. Northwest spoke in a series of grunts and screams and often yelled his trademark phrase: "I am going to eat this entire oak tree because I am a powerful wizard!""
The last frame of the opening sequence depicts Bill Cipher◊ surrounded by a ring with symbols on it. Every major character except Gideon and every important object in the series thus far has one of those symbols on him, her, or it, hinting at some sort of prophecy involving them all with Bill connecting them together. There is also an encoded message on the right, a Caesar Cipher saying STAN IS NOT WHAT HE SEEMS.
In the first season finale of Beast Wars, if you go frame by frame in the second-to-last shot of the explosion, the one where the wreckage flies past the camera, you'll see that included in that wreckage is Optimus Primal's chestplate and head.
In the Rugrats episode "Cuffed", when Angelica frantically tears open the package to get the toy handcuffs, you can actually see her casually toss away the key that she and Chuckie spend the rest of the episode trying to locate after they get cuffed together.
Danger Mouse on the Orient Express has a scene where the famous train is passing signs of European cities it is going through, the signs passing head on to the left and right in turn. One of the signs is the sign for the Willesden Green tube station.
In the Real Monsters episode "Hooked on Phobics", Ickis is humiliated in front of the class after getting injured on a scare assignment. He packs a suitcase full of comics and prepares to run away. If you watch closely you'll set that in addition to the comics he grabs, Ickis also packs "Shnookie", a sponge he used as a security blanket in "It's Only a Movie".
A subtle Continuity Nod can also be seen in the background of Ickis, Oblina, and Krumm's dorm room. A picture from an old vacation hangs on their wall, showing the monster trio in Egypt. Ickis wears glasses in the picture, just like he did in the pilot episode, "Monsters Get Real".
In one episode, Winslow gives Cat and Dog tests to see what they like and hate. At the bottom of the list is "laissez-faire economics" (which Cat likes) and "a welfare state" (which Dog likes).
In "Hotel CatDog", one of the vehicles outside CatDog's house reads "Willie Nelson Medicare Tour."
In the special The New Misadventures of Ichabod Crane, when Velma Van Damme the witch turns into a dragon and Ichabod casts a counter spell on her that gives her a second head, in the frames before she grows the second head, a pair of bare breasts appear on her dragon chest for a couple of frames.
In season 2 episode 14 of Wakfu, while the girls are taking a bath, Amalia's chest briefly dodges the foreground cover.
Watching this cassette illegally? Well, we’re watching you. Legally. Remove your grubby fingers from the play and record buttons. We hope your mothers raised you better than to be little thieves. All copies, dubs and bootlegs will be confiscated. You will be arrested and subjected to prison swirlies. Wipe that smirk from your face. There are no VCRS in jail.
In the Roger Rabbit short "Trail Mixup" after Roger gets his head caught in a beehive and starts spitting out the bees, if you pause this scene frame by frame you can see that the bees are caricatures of other Disney characters including Mickey Mouse, The Genie from Aladdin, Evinrude from The Rescuers, and Tinkerbell from PeterPan.
Pahkitew Island contestant Scarlett's audition tape mixes this with Foreshadowing. If you look in the background of her bedroom, you can quickly see a globe hanging from the roof... With several knives sticking into it.