When Richard Gordon and his butler James start the training fight while being held captive at the Frog's, there is suddenly a carpet on the ground, covering the mattress that softens James' fall. The carpet even covers the lower part of the door. In the next shot, the carpet is gone.
All the cars are left-hand drive - even the English police have no right-hand drive cars.
When they watch the film that Richard Gordon shot in the nightclub, the lower reel of the projector is not moving. It should be, otherwise the film already passed through the projector would be dumped on the floor.
When inspector Elk/Hedge is riding in the Double Decker Bus near the beginning of the film, the signs near his window are mirror inverted, indicating a flipped shot. They obviously shot the scene (in front of a very obvious rear-projection) as if driving from left to right but found out that it didn't intercut well with the outside shot of the real bus going the other way.
When the knife is thrown at Richard Gordon, in both close shots you can see the wire that was used to guide the weapon to the tree trunk.
About 14 minutes into the film a street sign with directions to London and Salisbury is shown. Salisbury is mis-spelled "Sallisbury".