St. Lucifer - S1-E11
Continuity mistake: The way Malcolm holds the gun when showing it to Dan changes between shots. Note his grip. (00:06:30)
St. Lucifer - S1-E11
Continuity mistake: Just after saying the widow could hold the charity event at Lux, Lucifer has his hands clasped. As the camera changes angle changes however his hands are apart. (00:07:30)
A Priest Walks Into a Bar - S1-E9
Continuity mistake: When Malcolm asks Amenadiel if food tasted this good before he died, his hands are clenched into fists with one finger pointing at the food. When the shot changes his hands are open and just facing forward. (00:39:20)
Continuity mistake: When Lucifer hands Linda the leaflet in the carpark, the way he holds it changes between angles. In one shot he's holding the sides and in the next he's holding the top. (00:25:40)
Favorite Son - S1-E6
Continuity mistake: When Dan confronts Mazikeen over leaving him in Chloe's apartment, he has his arms outstretched in the front shot and down by his side in the reverse shot. (00:21:20)
Sweet Kicks - S1-E5
Continuity mistake: The position of the guys Mazikeen attacks changes between shots. Note the woman Chloe knocks out. (00:31:30)
Continuity mistake: When Lucifer is walking towards the cowering Lindsay, she has her gun in her left hand. When the shot changes to behind Lucifer, the gun is now gone. (00:34:20)
Lucifer, Stay. Good Devil. - S1-E2
Continuity mistake: When Lucifer shows his devil face to the preacher at the start, the mascot behind Lucifer changes between shots. (00:03:00)
A Priest Walks Into a Bar - S1-E9
Continuity mistake: The position of the priest after he has died changes between when Lucifer walks over to the other person and when Connor walks over to the priest. (00:35:00)
A Priest Walks Into a Bar - S1-E9
Continuity mistake: When Lucifer goes to speak to the priest on the balcony after finding out about Connor Lucifer has his right arm by his side as he walks out. When the shot changes to outside his right arm is bent at the elbow and in front of him. (00:28:30)
A Priest Walks Into a Bar - S1-E9
Continuity mistake: When talking to the priest in Lux, Lucifer's hands are constantly changing positions between cuts. (00:24:20)
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 ★