A Sentimental Journey - S1-E19
Revealing mistake: Dialogue in the episode specifically mentions the A2 and A282 roads near Dartford, Kent - the actual roads used were the A1 Barnet by-pass, Galley Lane and Well End Road. The error is revealed by the sight of a road sign that appears just as the Austin Princess Limousine turns on the the main road - the sign shows that the car is not turning on the the A2/A282 near Dartford, but the A1 at Barnet some 40 miles away.
The Stand-in - S1-E9
Revealing mistake: When Pufnstuf tries to reason with Witchiepoo, Witchiepoo hits him with a giant powder thing and shouts "Make up." When Pufnstuf falls over his head almost disconnects to his body.