When the vicar hangs up after speaking to Mainwaring we hear the 'ting' of the vicar's phone through Mainwaring's. This would not be possible as the connection would have been broken.
When Wilson arrives in Mainwaring's office, Mainwaring reproaches him for being late. Wilson tells him "It's only twenty to ten." However, the clock on Mainwaring's mantle reads 9:15.