During the entire series, Charlie is only seen behind the wheel of a car once, and during that instance (in "The Maureen Ponderosa Wedding Massacre") Mac comments that it's been a long time since Charlie has driven a car. Not much else is made of it, but Charlie lacking a valid driver's license would make perfect sense— it'd be pretty hard for someone who's nigh-illiterate to study for a driving test, and it would also impair his ability to read road signs and street names.
In "Who Got Dee Pregnant?", Mac invents the phrase "browning out", using it to describe a drunken stupor that's not quite a blackout. He gets upset that Dennis starts to use it, but Charlie argues that since Mac put it out there, the phrase is "public domain." One might not expect someone as uneducated as Charlie to know what public domain is, but Charlie is a great musician, which might be where he knows the concept of public domain from.
During the episode "Mac and Charlie Write a Movie", Frank loves Dennis' idea of filming an action movie that features full, uncensored scenes of graphic sex. This foreshadows the conflict that he has with the Gang regarding the Lethal Weapon 6 film they made together, in which he wanted to include a scene of himself having sex with a beautiful woman.
When Dennis gets a job at the Oldies Rock Cafe in "The Gang Sells Out", The Waitress admonishes him for breaking the rules, even though (earlier in the episode) she could be manipulated into ignoring breaches of conduct if Dennis was involved. When she confronts him, it's while he's doing shots with a bunch of college girls. The Waitress was jealous and lashed out because of it, forgetting that she should turn a blind eye so Dennis won't think less of her.
When Charlie first plays his song "Nightman" in "Sweet Dee is Dating a Retarded Person," everyone points out that the second half of the song sounds like a description of a man breaking into Charlie's room and raping him while he's asleep. A season 1 episode had already implied that Charlie has an uncle who may or may not have molested him, and the implications get worse every time the uncle appears.
The title is It's Always Sunny in Philadelphia but the opening credits show Philadelphia's sights at night. Right from the start, you understand the irony of the title.
Frank's gradual flanderization over time, including his degrading physical state, are to be expected within the world of the show. In his first appearance, when he's introduced as an upper-class businessman, he makes a conscious decision to embrace the Gang's grimy, depraved lifestyle because his wife divorced him and he's been depressed to the point of suicide about losing most of his money. He was pushed into his upperclass lifestyle by his wife, but after his divorce he decides that he's more comfortable being a debauched drunk. He gets worse and worse in each season as he goes deeper and deeper into the Gang's world.
In "Sweet Dee has a Heart Attack" Charlie insists that he's got boxes of mail addressed to Pepe Sylvia and Carol in HR, neither of which actually exist. He seems to be going insane, but we know that Charlie is nearly illiterate. Pepe Sylvia = Pennsylvania and Carol in HR = Care of HR. However, this is all countered when Mac insists that everyone he's talking about really does exist. Charlie's delusion might have been an earlier draft of the script, or it might be intentionally vague as to just what is going on.
In the season 6 premiere, Dennis says that Mac owns literally nothing in the apartment. Seems impossible, but then you remember that when Mac had faked his death in Season 4, Dennis went through the apartment and threw away everything of Mac's ("Goodbye Mac's shit, goodbye Mac's shit...").
In season 10, the Waitress says that she still has a flip phone and doesn't use the Internet at all. At first it seems like a joke about how horribly uncool she is, but remember, she's been the victim of a very persistent stalker for over a decade. It makes sense that a woman in her position would want to stay as off-the-grid as possible.
"Mac Day" states implicitly that every member of the gang gets their own day to decide what is done. That means there is a Charlie Day. Who plays Charlie? Exactly
In "High School Reunion," an astute viewer might realize a few seconds early that the dance scene is a fantasy sequence because their choreography incorporates Dee's backbrace - which she was clearly not planning to wear that night.
"The Gang Turns Black" doesn't include any scenes in Paddy's or Charlie/Frank's apartment. All we get to see is Dee's apartment, Dennis' car from the outside, the street, the police station, a public electronics store, under a bridge and a nursing home where apparently Old Black Man actually belongs. All places that Old Black Man could have seen.
In "Sweet Dee Gets Audited" there is a debate between the characters about what to do with the corpse of a dog that is rotting in the alley behind the bar. Mac and Charlie want to perform a 'dog funeral', while Frank and Dennis want to throw the corpse in the dumpster. Mac probably supported the dog funeral because other episodes have shown that he still owns an old dog that he is emotionally attached to, so it makes sense that he wants to bury the dog with dignity. In addition, Mac is a hardcore Catholic, so it makes sense that he would place so much value on a proper burial. Alternatively, Frank has expressed the opinion that a dead body is essentially trash, to the extent that he does not care what happens to his own corpse when he dies. Dennis has expressed the opinion that eating a dog is no more morally wrong than eating a chicken.
From what we know about their childhood; Dennis and Dee clearly needed psychiatric help from an early age. Aside from carelessness and apathy; it is possible that Frank did not get them help because he didn't want them to get shanghaied to a nitwit school they same way that he did when he was young.
So Dee's neighbor Gary was the serial killer. That explains how he knew the shears would cut though bone. The gang insists that Dee doesn't fit the victim profile, but he's clearly attracted to her. Was he just biding his time? Did she really not fit the profile well enough to murder? Did he kill all those other people because Dee was taking advantage of him?
Given that the victim profile was young blonde women, is it possible that the Waitress could've been next?
In "Being Frank" it's revealed that Frank has an unknown terminal illness, but it isn't treated because Frank runs out of the hospital in a hurry. In "The Gang Goes to a Water Park" Frank pretends to have AIDs in order to get onto all the rides. This culminates in him bleeding in the pool, causing everyone to think he has AIDs. After this happens, Dee, Mac, and several kids fall into the water. Now, we know they don't have AIDs, but what if they now have that terminal illness Frank was diagnosed with? We don't know if it's bloodborne or not.