Deliberate mistake: The number of clicks (sounds) Christine made on the "text to talk" device did not match the total characters she typed. With few exceptions, there were a lot less clicks/sounds than there should have been based on the amount of letters/words she typed. (00:06:40 - 00:15:30)
Deliberate mistake: Except in certain angled views of Fred (with Matt in the passenger seat), the rearview mirror is darkened/blackened. (00:08:55)
Deliberate mistake: Considering the position of the surveillance camera (height and angle - almost parallel to the side of the building as well as to the sidewalk), plus the lens being indented within the outer frame, it would not have been able to capture a view of Redd on the sidewalk below like it did. (00:24:40)
Deliberate mistake: One of the first shots in Greece shows Benoit Blanc and Lionel standing on the pier. As director Rian Johnson himself says in an interview for the Vanity Fair Youtube channel, "Matches nothing else in the scene lighting-wise, but it's gorgeous soooo...who cares." (00:13:30)
Deliberate mistake: "Mama's Home Cooking" - a wood frame building in 1850 - was on fire in multiple spots (though not yet engulfed). The townsfolk lined up to pass a bucket of water to douse the flames, which would be futile given how involved the fires were, but they were able to extinguish the fires and the wood frame building was largely intact and the boards did not look charred. The building should have been smoldering hot for some time, but Angel was able to sift through the rubble to salvage some items. (01:45:58 - 01:46:56)
Deliberate mistake: Bill's back is some distance from the middle gap in the curtains, but an exterior view of him hugging Ellen captures their faces as if they are standing closer to the window and the gap in the curtains is wider. (00:32:45)
Deliberate mistake: The thug wearing headphones and facing the money on the shelves did not notice that Savage forcefully opened the door several feet to his right, slamming it against the adjacent wall, or that Savage had entered the room. The bang to and vibration from the door hitting the wall should have been enough to overpower the sound from his earphones, but - even if they weren't - his peripheral vision should have sensed the door opening and Savage entering. (00:02:55)
Deliberate mistake: Richard falls (or is pushed) into the lake, and a loud splash is heard. However, as he pulls himself onto the deck, no water drips from him. But there is already a large wet area further onto the deck when the sheriff tosses him. (00:18:05)