Creator Backlash: Mandy Patinkin has gone on record saying he considers his tenure on the show the biggest mistake of his acting career, due to how disturbing he found its content, despite critics widely praising his performance.
Several actors who played bit parts/recurring roles on Supernatural are minor characters in this show. "The Thirteenth Step" has Jess as one half of a young couple on a killing spree, Lilith's child vessel was abducted to be Stockholm Syndromed into a twisted Roma marriage in "Bloodlines", Lucifer's second-choice vessel was a cop in "Brothers in Arms", and Rufus was an unhelpful police chief in "Legacy."
Recycled Script: Billy Flynn's origin is nearly identical to Frank's, and he has a lot of similarities to Karl Arnold/The Fox.
What Could Have Been: The show was originally going to be called Quantico and Gideon was originally named Jason Donovan. An early version of Hotch was described as a "blond haired, blue eyed Mormon" and Lukas Haas (the guy who played The Footpath Killer) originally auditioned for the part of Reid, and might have even got it if he hadn't begun expressing doubts about doing a series.
Writer Revolt: In "JJ", listen very closely to the dialogue. AJ Cook was let go from the show for purely financial reasons, a decision that the cast and crew obviously reviled, and the subplot is about how her promotion is being forced by "people above their pay grade" (the network). She really lays into them in her closing monologue in place of the usual ending quote.
Considering how the writers, the cast and the entire fanbase hatedwhat the network executives did, there is a general consensus that the Take That against them was entirely justifiednote and cathartic (and one of the few times when a Writer Revolt has been fully supported by more than just fanbase splinter groups).
Fortunately there is a happy ending to all this as Cook became a series regular again at the start of season seven. The main reason she was let go was so the execs would have money to do that spin-off, but it got cancelled after only 13 episodes. With their funds now freed up, CBS soon rehired Cook, perhaps knowing that there would be a good chance of a sharp drop in ratings if they didn't bring her back.