Revealing mistake: Early in the episode the dead body in the barn ever so briefly flutters his eyelids.
Factual error: You cannot remove a kidney laparoscopically. The wounds were way too small.
Unquiet Mind - S2-E1
Character mistake: When Walt is coming up to Mars cabin he hears a gun shot, he then re cocks his gun ejecting a live round, since he has not yet fired it. (00:20:30)
Goodbye Is Always Implied - S6-E10
Continuity mistake: Near the end, when Vic is on the porch bench, she's holding a mug in her hand. When Walt goes to sit down, she's still holding it, but in the next shot her hands are empty and the mug is on the porch railing.
Answer: Chance.