At this point in history, it's pretty jarring to discover Dennis blasting "Never Gonna Give You Up" in his car is not a reference to the Rickroll... because it pre-dates the meme by two years.
In the episode "The Gang Saves The Day", Charlie's fantasy is a parody of the Disney/Pixar movie Up. 6 years later, Always Sunny and Up are now under the same roof at The Walt Disney Company following their purchase of 21st Century Fox, the legal owners of Always Sunny.
A year after the episode "The Nightman Cometh" premiered, there was a movie released that bore a striking similarity to the titular play.
A prominent part of Charlie's America song in "Charlie Goes America All Over Everybody's Ass" is the words "rise up". Bonus points for Charlie singing them basically the same way Lin-Manuel Miranda does in Hamilton's death song.
"The Gang Solves the North Korea Situation" had Mr. Kim as a Crosscast Role four years before Margaret Cho played Kim Jong-Il on 30 Rock.
In "Chardee MacDennis 2: Electric Boogaloo", Frank's team flag resembles the flag of Nazi Germany but with the swastika made up of for capital 'F's. A similar flag is used by the Facebook Polandball community where the swastika is made up of four 'F's in the same style as Facebook's logo note This was used as a Take That! towards Facebook's reputation of banning Polandball pages as well as their No Swastikas rule.
"Charlie Has Cancer" features a flashback where Dee approaches Mac with mistletoe only to get punched in the face by him. This was in Season 1, a full year before their actors started dating in real life.
"Charlie Catches a Leprechaun" has Dennis' attempt to implement a "mobile bar" to pick up customers and serve them beer on Saint Patrick's Day failing because he insists on an online pay system through social media rather than letting the customers pay cash. A few years later, mobile pay systems such as Venmo and Apple Pay became mainstream and caused cash transactions to decline, so Dennis' Complexity Addiction ends up coming off as his idea just being ahead of its time.
At one point, Mac compares Dee to a fish because "her eyes are so far apart, like they're on both sides of her head." Kaitlin Olson then played a whale shark named Destiny in Finding Dory who fits this exact description.
Dennis dismisses Frank's interest a woman on the flight in "The Gang Beats Boggs" because she's "North Dakota trash." "Dennis' Double Life" reveals that Dennis had a one night stand and fathered a child with a woman from North Dakota as a result of the events of "The Gang Beats Boggs."
Charlie's mom not finding Dennis attractive and refusing to sleep with him in Season 2 is even funnier when later seasons depict her as someone who will bang just about anyone.
In "Who Got Dee Pregnant?", Charlie trades his Phantom costume for Dennis' Luigi costume at a Halloween party. Charlie's "Green Man" persona and the scene of him asking the spa about its "spaghetti policy" then became the subject of many jokes after the announcement that Charlie Day would be playing Luigi in the Illumination-animated Mario movie.
The pilot has Dee's friend Tyrell talking about gay sex in a way that only Dennis picks up on. Mac assumes that Tyrell is talking about beating a guy up and can be heard saying, "I would like to do things like that!" Rob McElhenney has admitted that they hadn't yet planned on making Mac gay in Season 1, making Mac's line in the scene fall squarely into this trope.
In "The Gang Goes To Hell", Mac puts battery acid on a crucifix he gives to Frank in a ploy to make him consider converting. The Gang then imply that he might actually be the Devil, something Frank leans into out of amusement. Funny enough, Danny DeVito would actually go on to play Satan in Little Demon.
In "Frank Reynolds Little Beauties", Frank wants to make sure he isn't seen as a child molester after buying a child beauty pageant by wanting to sing about how he doesn't "diddle kids". The others criticized Frank about how wanting a song about not diddling kids are going to make other's think he does do it. This becomes darkly hilarious after Colleen Ballinger was accused of grooming young fans and she denied the allegations by posting a video singing about how she didn't groom any fans. There are fans who have link the similarities.
In "Mac and Charlie Write a Movie," there's a Running Gag that Dennis is spending all of his time typing on his phone, which is supposed to be inherently silly because smartphones hadn't quite yet become ubiquitous. The phone that Dennis is using is a Blackberry. Glenn Howerton would go on to star in BlackBerry, charting that company's role in the ubiquity of smartphones. Behind the scenes, Howerton confirmed that he was unimpressed by the phone that his later movie would portray as garbage.
In "Who Pooped the Bed?", Mac and Dennis' complaints about Sex and the City being a long show that won't die come across as ironic now that this show has been running for almost twenty years.
Most of Schmitty's scenes in "The Gang Gets a New Member" feature him throwing an imaginary lasso, which was just seen as a funny little character quirk at the time. Now, however, it's led to several jokes and references to Jason Sudeikis' othermajor TV show.
about his big, masculine (and totally real) hands in "McPoyle vs. Ponderosa: The Trial of the Century"? Funny. Donald Trump almost quoting him verbatim