A. B. and C. - S1-E3
Revealing mistake: In the fight at the end of the B scenario, Number 6 has a small problem. His pants split open in the back. Fortunately, they somehow manage to mend themselves again.
The Big Dog - S2-E11
Revealing mistake: In the open scene, Gannon takes a pill and hand-cups it into his mouth. As he removes his hand, you can see the pill bounce off his cheek, down his jacket to the floor, but he acts like he took it.
Task Force - S2-E16
Revealing mistake: The body of the alien is obviously a dummy after it falls out the helicopter.