Revealing mistake: When Bill is stabbed, the pad beneath his shirt - where the knife is stabbed - protrudes.
How To Make a Killing Without Really Trying - S6-E14
Revealing mistake: When Jessica enters Gordon Tully's office and looks around, there's a photo of him with Buddy Black. The picture is an awful, poorly done composite: the lighting on their heads doesn't match the rest of the image, the head sizes are out of proportion with the bodies, and the overall light direction is inconsistent.
Revealing mistake: When the first scene at Emma's hideaway was shown, obviously the two characters Emma and Jessica were filmed seperately on the couch and then the film was put together (as both are played by Angela Lansbury). However Jessica did a lot of moving while she was talking, so Emma was looking straight across at where she had been, despite her bending down while she talked.
Tinker, Tailor, Liar, Thief - S8-E15
Revealing mistake: In the second close up shot of Nigel Atkins lying dead, after Jessica drops her Harrod's bag in the chair, it is obvious he is still alive as you can see his pulse on the side of his neck.
Deadly Lady - S1-E2
Revealing mistake: When Howard Duff's two daughters enter the police station to identify his body, the cop pulls back the blanket. Howard's eyelids are moving even though he is supposed to be dead.
Revealing mistake: When Danny recounts crashing the pickup and breaking the tail light, you can hear the light shatter when he hits the tree - but it doesn't actually break. In the next shot, however, the tail light is suddenly smashed.
Town Father - S6-E11
Revealing mistake: When the two men knock on Jessica's door to inform her about the murder, the landscape behind them is a painted backdrop.
Jack & Bill - S6-E5
Revealing mistake: When the episode ends and Bill leans down for the dog to lick him, a shiny light-brown spot is visible on the actor's jaw - seemingly a bit of food placed there to prompt the dog's trained reaction.
Programmed for Murder - S8-E18
Revealing mistake: In the outside shot of the hospital, just before Mort discusses his cough with Doc Hazlet, you can see a sign saying the hospital is located in Ohio, not in Maine. This isn't the only episode where this exact mistake is made.
Revealing mistake: When Miguel tries to sail away, Jessica runs towards the shore. When she enters the water, Angela Lansbury is replaced with a thinner and younger double.
Revealing mistake: When Santana is killed, a rectangular pad where the arrow was stabbed protrudes under his shirt.
Revealing mistake: When Cagliostro is stabbed, there's a protruding rectangular pad under his shirt where the knife is stuck.
Revealing mistake: When the episode ends and Jessica drives the buggy, she is replaced by a very obvious stunt double with a wig that not only does not resemble her hairstyle, but it's grey as opposed to her light blonde colour.
Revealing mistake: Kat pumps her shotgun several times with no shells coming out.
Revealing mistake: All the shots of snow falling, mainly when Gunnar is about to be killed, are a very obvious crappy footage of flakes falling projected over the footage of the show. Not a single flake falls on the objects nor the people.
Something Borrowed, Someone Blue - S5-E9
Revealing mistake: When the corpse lies on the bed the pad under the clothes where the thermometer is stuck protrudes.
Trevor Hudson's Legacy - S5-E18
Revealing mistake: There are 2 scenes involving Jessica inside a car. In the first one the car drives calmly as seen from the outside shots, but from the shots inside the car is speeding. When the episode is about to end the car speeds to the police station and now the exact same projection is played, though this time it makes sense.
Jack & Bill - S6-E5
Revealing mistake: In the toilet scene, when Max slams the terrorist against the wall, he is replaced by a very obvious stunt double with longer hair and a noticeably larger build.
Revealing mistake: When Jessica visits the basement, there's plenty of light everywhere. She then turns off the light, but there's still plenty of light. However, when Adam Frobisher shows up half a second later, the whole place is now in darkness to allow for all the mystery and the intrigue.
Revealing mistake: When Emma hits a man using the flower pot, Angela Lansbury's stand-in is a much younger stunt double.
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.