Doctor Horrible's Sing-Along Blog. You could watch Penny and Hammer on their date scene, or you could look in the background and see Doctor Horrible stalking them in a hilariously inept manner, even pretending to be a soup kitchen worker.
The first Nerd vs. Critic battle. After the Nerd chastises the Critic for not shutting the door after coming into his house, just before the door closes, Toucan Sam flies by.
Any episode of The Joker Blogs is sure to include many blink-and-you'll-miss-it inside jokes about the DC universe. This comes to its logical conclusion during episode 18, which was in the form of a TV news broadcast. The news ticker at the bottom of the screen consists of nothing but comic book in-jokes (over 25 different Batman villains/allies are referenced), to the point that it mught be necessary to watch the video twice — once to watch the video and the other time to watch the news ticker.
Used often in Djy1991's version of Half-Life: Full Life Consequences. One example includes someone in the background randomly firing cans around with the Gravity Gun, whilst John Freeman is reading the email from his Mom.
The third and fourth videos, Hero Beggining and Free Man, are filled with this, often involving the GMan.
Thanks to the Machinima nature of Red vs. Blue, you might be able to catch some of the cast wandering aimlessly around the set while other characters are talking in the foreground. The most memorable is probably Agent Washington using quite a lot of explosives to dispose of a recently deceased fellow agent's body.
When Church is getting the others to fix up his legs, and in the background the red's jeep which he is unknowingly controlling is going everywhere. Soon after it almost kills Sarge.
Unintentional Funny Background Events occasionally occurred in older episodes when characters not currently in use ended up in a shot. Perhaps most notable was a brief glimpse of a blue soldier who looked like Caboose in an early episode, declared to be Caboose's evil brother Cecil, who returns to kill Caboose in one of the alternate finales for season 5.
During the Cinema Snob review of Nudist Colony of The Dead his cat wanders onscreen and watches as he throws a crumpled up newspaper across the room.
The video "Brad Tries...Durian Toffee" could just as plausibly be titled "Brad's Cat Has Fun", given that it hops around on the couch behind Brad, falls down behind it, gets out, and falls back in multiple times.
In "Brad Tries Jones Soda Christmas Packs", the cat has her own show in the background.
In "Brad Tries Sperm", Brad has to caution his cat against sniffing the curiously-named canned cocktail he's about to consume, as that would just make the video all kinds of illegal.
At the end of the Rock: It's Your DecisionDVD-R Hell review, Brad describes just how messed up the movie was, and how it detailed the utter ruination of the main character as he is brainwashed. As he states that he is sure that, ten years later, the main character would hang himself, Lloyd picked that EXACT moment to jump up onto the couch as Brad smiles.
In one episode, Lloyd starts meowing quite loudly off-screen, to which Brad reacts by assuring the cat they will drink the drink, which shuts the cat up.
This video where the main focus in on the Spy trying (poorly) to do Caramelldansen, while people in the background are doing normal things (at least, normal for Team Fortress 2). However, about a third of the way in, an Engineer spawns in on top of the Spy and starts dancing, before the cameraman kills him. Then, about two-thirds of the way in, another Spy, who has, until this point, been just running around shooting people, decides to get in on the act, and actually does a much better job of Spy Crabbing in the background.
In one video of The Tourettes Guy, Danny screams PIIIIIIIIIIIIISS! so loud into his phone, a page on the calender behind him falls onto the previous month.
In Zero Punctuation's Super Paper Mario review, when Yahtzee shows the Dictionary.com definition of "game" (while complaining about padding), the other tab in his browser is titled "Super Huge Tits".
In the episode of Monster High, "Ghostly Gossip", just at the beginning there is a blink and you miss it moment where behind Cleo, a student is being attacked from her locker by tenticles similar to the image above.
Project Million: After receiving a Mind Screw from The Wire, Spazz spends several minutes in the background trying to make sense of it.
Ever since the Spoony One got a new dog, you can see her in the background having fun in a number of the V Logs.
A lot of the contributors to Kickassia seem to be trying to out-mug each other, making group shots hilarious for picking out different facial expressions in the background.
They keep it up in Suburban Knights. One notable scene is when one group has to search for the next clue, which is hidden in Elisa's basement. As Brad spins a story about them being a group protesting terrible stage shows, you can see the rest of the group behind him giving hilarious hammy grins and head-nodding. The scene later gets some laughs from the reviewers who did the commentary.
And later on, when Benzai stops to pray to Crom, look in the background. First you'll see Jaffers chasing Critic with a sword. Wait a few seconds. Then you'll see Critic chasing Jaffers with the same sword, having somehow gotten it off-screen.
Ever since Doug Walker started doing vlogs of Adventure Time with his friends Jason and Jori Laws, their cat Naruto has been making appearances on par with Lloyd and Oreo.
JonTron has unscripted ones in the form of whatever Jacques the parrot is doing for that particular shot, be it taking a flapping leap off elsewhere, weave around as Jon hams it up, or simply nibble on stuff, ears included.
In the stinger for Episode 8 of RWBY, crime lord Roman Torchwick is seen poring over his next heist. If one looks closely at the map behind him during the scene, they can see that he's labeled a section of it with "Stupid Cops."
In Episode 1 of season 2, while Sun and Neptune talk, a massive Food Fight erupts behind them without them noticing. Jaune gets thrown into a window and slides down it.
Later in the same episode, there's Roman's extremely mature expressions when Cinder is chewing out Mercury and Emerald for going after Tukson.
Speed Demo Archives runs the Awesome Games Done Quick charity event, where live speedruns are attempted to raise donations for charitable causes. Speedrunner sinister1 plays the original Punch-Out!! on an actual NES, no mean feat for a game that is already Nintendo Hard. The thing is, he's playing it blindfolded so he has to rely solely on audio cues and pattern memory to play. He actually goes all the way to Mike Tyson himself while blindfolded and using no continues. The editor in charge of the stream sneaks in his own commentary on this feat in the middle of the Super Macho Man fight—he changes part of the run's caption from "Estimate: 1:00:00" (meaning sinister1 is expected to finish the game in an hour) to "Estimate: Jedi."
The zombie parody How To Survive A Zombie Apocalypse has instances of this when a member of a survivor group spills a soliloqy about life while her partners fight like hell to not ger mauled by zombies in the background.
In 20 Haunting Halloween Facts by Matthew Santoro, Matthew says, "I mean, come on, ghosts? They don't exist! Never once in my life have I ever seen a ghost. And I never will." While he's saying that, a ghost walks in the background.