Revealing mistake: Whenever any of the characters are in a moving vehicle talking to one another, it's pretty obvious that they are in fact in a stationary vehicle on a green screen. The green glow around their heads is visible.
Lucifer, Stay. Good Devil. - S1-E2
Revealing mistake: When Jimmy grabs the cell's door, in the shot that happens before he starts banging his head on it, the supposedly secure door gives way effortlessly, showing big gaps. (00:42:30)
Favorite Son - S1-E6
Revealing mistake: When Lucifer is going through the forensics bag next to the dead body at the start, the body of the victim can be seen breathing. (00:07:30)
Favorite Son - S1-E6
Revealing mistake: When Lucifer takes off in Chloe's car during the stakeout, tyre marks are visible in the gravel from previous takes. (00:32:10)
Revealing mistake: When Lucifer convinces Ricky not to jump off the building, it is blatantly obvious that the two actors are on a green screen and not an actual rooftop. The green glow around their heads is visible. (00:13:10)
St. Lucifer - S1-E11
Revealing mistake: When Chloe is getting dressed at the start of the episode, she is wearing a flesh colored bra underneath the bed covers that she has wrapped around her when she mentions her ex dumping her by text.
Liar, Liar, Slutty Dress on Fire - S2-E2
Revealing mistake: When the cops find the body in the hotel room, the actor playing the body is visibly breathing when he's supposed to be dead.
Answer: Boo Normal and Once Upon a Time were going to be part of season 4, then the show got cancelled (before being picked up elsewhere). As such they were added on to the end of season three as "bonus" episodes, hence the continuity being off. But not really a mistake, just a quirk of broadcasting. Boo Normal was originally going to be between the episodes "High School Poppycock" and "Infernal Guinea Pig" in season 3 before being slated for season 4, presumably because aside from the point you've raised, it was fairly flexible, continuity-wise.
Jon Sandys ★