Visible crew/equipment: When Chloe shuts the door on Dan, a stage light is reflected in the glass of the door. (00:04:10)
Continuity mistake: At the end of the kitchen scene, Dan turns to tell Chloe he is disappointed in her. When he does this, in the shot from behind Chloe looking at Dan, Chloe has her hand on the door frame at head height and is leaning against her hand. When the shot changes to behind Dan looking at Chloe her hand and head have moved instantly with no time to do so. (00:04:00)
Continuity mistake: During the kitchen scene in the beginning, the number of pictures on the fridge keep changing in various shots. (00:03:00)
Continuity mistake: When Chloe is kicking Lucifer out of the kitchen at the start she has her hands on her hips in one shot and down by her sides in the next. When Lucifer finally admits defeat and says "Fine" (00:04:00)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: As Lucifer is leaving Linda's office, she has hold of her shoe up by her chest. When the shot changes it's now in her lap. (00:40:05)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: At the end of the episode, the therapist in the first part of the conversation (before she gets her heels) holds her right hand on her bosom in the close-ups, while it is lowered in the wider angle. (00:40:10)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: Lucifer enters the conference room of the "rebranding" of Ty. You can see Ty turn around in his chair, but after the cut he is still leaning against the table, performing the turn again a second later. (00:35:30)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: Joe rubs his protege Ty's shoulders saying "But we kept it out of the press." Ty has his hand raised, while in the shot before he had it on the desk. (00:35:15)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: Lucifer looks at the file of Ty's ex and isn't holding the file in one shot but is in the subsequent shot. (00:21:00)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: Ty's agent places the folder down for Lucifer and Decker to look at. The position of the folders changes in subsequent shots. (00:20:50)
Continuity mistake: At the start when Trixie comes running in to give Lucifer a hug, Lucifer has got a jar in his hand. As the shot changes to him actually being hugged, the jar simply vanishes from his hand. (00:03:05)
Continuity mistake: At the beginning of the episode, a cop pulls Lucifer over. The car stops with the front wheels on white road markings. When Lucifer is shown taking off again, the street is just not the same, there's no white horizontal lines, and a manhole appears under the car. (00:02:45)
The Would-Be Prince of Darkness - S1-E3
Continuity mistake: Chloe takes the phone in Lucifer's hand as he is watching the registration. Depending on the angle Chloe has her right hand on the phone itself or off it, signaling Lucifer to stay put. (00:13:15)
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 ★