Wallis is showing his idea to the doctor and draws on the sketch of the dam. Shortly after, he shows the doctor the same sketch again and the middle line we saw Wallis draw has now changed position.
A (corrected) entry states that a USAAF B-17 'Flying Fortress' bomber is visible in the film. In fact in some prints produced for release in the US irrelevant stock footage of a B17 was randomly cut into the film to make it more acceptable to American audiences. This was a deliberate alteration of the film done very much against the wishes of the producer and director, and it explains the 'error' - the contributor has seen one of the non-canon US prints.