Continuity mistake: The lighthouse keeper's pistol keeps changing distance from Sgt. Troy's head between shots.
Continuity mistake: Over the course of many Dark Shadows episodes set in the year 1795, Countess Natalie Dupres' dark, distinct, three-dimensional facial mole changes sides from left jaw to right jaw, briefly vanishes altogether, then returns as a faint, painted beauty mark on her right jaw.
The System - S3-E15
Continuity mistake: Phelps takes the tape from the recorder's right-hand spindle. When he opens the acid vat, we can see the tape he just removed back on the recorder - but on the left spindle. The tape he then drops into the acid doesn't remotely resemble the one he pulled, and when he drops it, the recorder's left spindle, where the full tape was in the last shot and where the empty reel should be, now has nothing on it at all. (00:02:50)