Channel Hop: In an effort to attract interest in FX's new comedy-oriented spinoff channel FXX, Sunny will air its 9th and 10th seasons on the new channel. It's also worthy to note that The League and Legit will also make the hop this fall (Archer was supposed to go to FXX as well, but, instead, stayed on FX and went from airing on Thursdays to airing on Mondays). Read more about it here.
Jossed: A fan theory is that The Waitress's name is Nikki Potnick because she couldn't find her name tag at her high school reunion and Frank had stolen the "Nikki Potnick" tag. During his "Ask Me Anything" for Reddit, Glenn Howerton said in no uncertain terms that "the Waitress is NOT Nikki Potnick."
And why does Mr. Larritate have a refrigerator filled with the severed heads of multiple women?
Mr Belding (Dennis Haskins) is a gym teacher who's been accused of sexually molesting boys?
And why is Kirby married to a post-op transsexual (who was a Wakefield twin [on the TV version of Sweet Valley High] and a bisexual girl named Sophie [on the short-lived That 70s Show spin-off That 80s Show])?
Dennis (Glenn Howerton) was Cory on That 80s Show, a short-lived spin-off of That 70s Show. Brittany Daniel also appeared as a bisexual girl named Sophie on the show, and Charlie Day appeared as an extra talking on his cell phone in the pilot.
Danny DeVito's daughter Lucy has had a couple of one-shot cameos in "Mac bangs Dennis' mom" and "The Gang find a dumpster Baby".
Miscellaneous fun facts:
When series creator Rob McEllehny was asked a question about his character Mac in a featurette, he replied "I don't know... I just show up, say my fuckin' lines, and go home."
Charlie's play The Nightman Cometh was for a while actually touring various cities around Philadelphia with all of the original cast members.
Charlie Day revealed that Charlie's rat-killing speech in "Charlie Kelly: King of the Rats" is basically how he got his role in Pacific Rim.
According to Charlie Day, during a recent interview on Conan O'Brien, Danny DeVito has stated that he's up for pretty much ANY storyline the show might throw at him save for one topic: prison rape. Apparently, DeVito informed series creator Rob McEllehny that was the only type of shock plotline that he refuses to have anything to do with.note Offhand jokes about prison rape are apparently fair game, though. In season six's "Mac's Mom Burns Her House Down", there's a joke about Frank thinking that prison rape happens at nursing homes.
The writers of the show, Game of Thrones were such big fans of the show that they decided to write an episode for It's Always Sunny called "Flowers For Charlie" (the 100th episode where Charlie is put under a medical treatment that makes him smarter).