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.
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.
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.