The doors of Freddie's bedroom closet have slats which are shown to slant downward from inside to the outside of the closet, but when Blaze and "Cosmo" are hiding inside, the slats are tilted more outside-in, but parted somewhat to allow light for filming. Before and after the shots of the two snoopers' faces, though, the closet doors are shown with the slats clearly closed.
When the murderer stabs Max Fuller, the blow comes in an overhand style, striking downward. But when we see Fuller's body on the floor, the knife is plunged straight in, as from a blow delivered underhand.
DCI Barnaby asks Ruth to write down all the numbers that might be used in the combination for the safe. He mentions several different kinds of numbers, but when Ruth begins to write the numbers, she has time for only two or three before she is interrupted by Jones' entrance. And Barnaby never asks her to complete the list. But later we see the list with at least six numbers written down.
When Barnaby enters the three successful numbers on the safe, he turns right (clockwise) for the first one, left (anticlockwise) for the second one, and then left again for the third number, which isn't how a three-number combination lock works.
When Susan Crane mentions tax avoidance, DCI Barnaby asks whether they were evading tax. These are two different things. In short, tax avoidance is legal, whereas tax evasion is illegal.