Tabula Rasa - S1-E3
Revealing mistake: With Marshal Mars in pursuit, as Ray drives his pickup, he holds the steering wheel with his left hand, and his stiff prosthetic right arm is down at his side. When Kate grabs for the steering wheel, in the side exterior shot the stunt driver is quite noticeable, as he grips and turns the steering wheel with his good right hand/arm. (00:30:40)
Revealing mistake: Season 5, episode 8, "My Name is Mac Taylor": When the killer sets himself on fire, he's not wearing any gloves. In the next shot he is, to protect the stuntman from the flames.
The Hands of Justice: Part 1 - S2-E50
Revealing mistake: Right before Chancellor Shepherd briefs the students about the tournament, there is a shot of Obelisk Blue and Ra Yellow students around the outside of the building. Later in the episode, after Zane gives his medal to Blair, the scene cuts to outside Duel Academy, and the exact same shot of the Obelisk Blue and Ra Yellow students has been repeated a second time. (00:02:48 - 00:08:45)