Revealing mistake: When the red Dalek falls down the shaft, there is a very clear shot of its base with three small furniture casters (though it must be a dummy, as there is no hole for the operator's feet to protrude).
Revealing mistake: When the Red Dalek and the Dalek Supreme fall down the mine shaft, they have been replaced with obvious models that don't resemble the Dalek's image (i.e. larger necks; more skirt balls; thinner dome lights).
Revealing mistake: When the workers are running away from the mine before it explodes, one of the female extras (the woman wearing the blue shawl) suddenly slows down to a relaxed stroll - obviously thinking she's safely out of shot.
When Wyler and Susan are escaping in the van, the windscreen is shattered and Wyler punches a hole for visibility. In all further shots of the van, the windscreen is obviously intact with the shattering merely cosmetic.