When in Lady Felicia's car outside the prison, Father Brown refers to the serial killer enjoying "sowing the seeds of doubt in the minds of his loved ones' victims", which makes no sense - especially as he is talking to one of the victims' loved ones. A transposition of words that should have led to another take.
When Brenda listens to the radio in the car, the BBC time signal is incorrect. The extended final pip marking the hour was not introduced until 1972. Prior to that the pips were all of the same length.