Deliberate mistake: There are no lights ever to be found in the underground world and the ceiling is literally the surface's ground, but there is still a perfect level of lighting regardless.
Continuity mistake: When Dr. Brockton is descending into the cave her hair changes from one style to another.
Other mistake: After the cricket match when Elspeth Wentworth runs away from her father, Mr Wentworth (Richard Wattis) calls her Kathryn, mistakenly calling the character by the actress' name (Kathryn Dawe).