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...").
It's increasingly demonstrated that Mac is Armoured Closet Gay, so in the season eight episode "Frank's Back in Business," you might wonder why he immediately checks out when he and Dennis mistake a caddie for a male prostitute. However, Mac is always obsessed with huge, muscular bodybuilders. Because the caddie is a young, slender boy, he's not Mac's type.
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
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?
Charlie's "Nightman" song. Since the show did drop hints on the possibility that Charlie got molested as a child the song could actually be about Charlie being sexually abused as a kid.