Revealing mistake: UFO - Timelash - in this episode the aliens with the help of 'turncoat' Turner have frozen a moment in time in which they will attack and destroy SHADO HQ. Everything and everyone is frozen in place - except for the guy in the background walking across the road during Straker's and Turner's 'car' chase in the 'Harlington/Straker' go carts.
Plot hole: "Psychobombs": The alien-possessed Mason steals Captain Lauritzen's fingerprints by pressing his fingertips to the captain's and "heat transferring" the prints. Big problems here: those prints would now be backwards and for the wrong hand. But when the computer later reads the prints, it fails to notice this and passes him through security as "Identity Correct."
Revealing mistake: "Computer Affair": In the interior shots of Mobile 1's cockpit, we see that there's a crew of 3 and that the pilot is black. But in shots of the exterior (miniature model stock footage), there's only one occupant, a pilot, and it's a doll with distinctively Caucasian features.