Mr. Garrity and the Graves - S5-E32
Deliberate mistake: Garrity blackmails the town citizens into paying him not to resurrect their unlamented "loved ones." Amazingly, every man in the saloon just happens to have hundreds of dollars ($500 - $1200 each) in his pocket to pay up with. In 1890 money, that's roughly equivalent to a group of modern-day bar patrons all carrying ten to thirty thousand dollars around in their pockets. The scenario was apparently used in spite of its incredibility just to more quickly advance the plot.
Mr. Garrity and the Graves - S5-E32
Revealing mistake: When the town's people are waiting for Garrity's arrival at midnight, we hear an old clock informing everybody it is midnight and they all turn around to see. But look at the time on the clock - you can quite clearly see that it is actually five minutes till 6 o'clock.
