At the end of the film, Calvert gives the girl a single brick of gold. She remarks that it is only one, and Calvert (straining to hold it) responds "do you know how much these bloody things weigh?!" However, earlier in the film, the deep-sea diver lifts three bars. This is physically impossible even for a strong man. If Calvert could barely lift one bar, then any diver would find three bars totally impossible, even allowing for the extra buoyancy that the bars would have in water.
During the customs inspection, the hijackers are dressed as customs officers. However, only one of them is wearing the cap badge of a Customs & Excise officer. The other is wearing the cap badge of a Royal Navy Officer. When this man leaves the boat (the other hijacker not being visible) his cap badge has now changed to a Customs & Excise badge.
The helicopter crash is strong enough to break the tail beam while Calvert who is not even wearing a safety belt (and is upside down in the cabin) is virtually unharmed. He would have broken numerous bones at the least.
When gunmen shoot at a helicopter which is supposedly crashing, the smoke disappears into their guns. This shows the film was run backwards and the helicopter was taking off. In the film's trailer (available with the DVD) the shot is run correctly.
Towards the end of the film Calvert fires a rocket-powered grappling-hook to help scale the cliff. This is clearly attached to a box of thin twine which is shown rapidly emptying as the rocket heads upwards. When the hook lands and catches onto the base of a cannon, the twine has magically evolved into a 1 inch thick rope which Calvert then uses to climb the cliff.