Character mistake: Chloe, Maze, and Ellen are discussing a murder and the death of Vernon Gill who was killed when hit by a car after he was fleeing pursuit from Chloe and Maze. They were discussing Dirty Doug who came in last night and Maze says "he came in last night and Gill said he threatened him for taking credit for Lee's murder." How could Gill say anything when he was already dead from being hit by a car. Should be "he said Gill threatened him" not Gill said since Gill's dead, in hell. (00:30:55 - 00:32:00)
Deliberate mistake: Lee's old crew of three are shooting various caliber handguns at Chloe and Lee's sister. Chloe took cover behind a kitchen island. Kitchen islands are made of thin wood, the only piping in them is plastic and some brass/copper. All of those handgun calibers would have penetrated the kitchen island. (00:45:45 - 00:47:00)
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 ★