This picture was planned as being the first of a series of spy movies featuring the character of Philip Calvert (in this film played by Anthony Hopkins). Around this time, it was known that Sean Connery would not be doing another James Bond film after Diamonds Are Forever (1971) and there was speculation that the Bond franchise might end. As such, a potential vacuum was sensed by rival producer Elliott Kastner. But when this movie failed at the box-office, plans for a film franchise to succeed the James Bond movies were scrapped.
According to the film's production notes, producers Elliott Kastner and Jerry Gershwin commissioned novelist Alistair MacLean to write two more Philip Calvert spy adventure screenplays but when this film was not successful at the box-office, these projects went into turnaround.
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.
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.
Philip Calvert: I have everybody breathing down my neck: the Admiralty, the Government, the Americans... and the insurance assessors. Grubby little men with gabardine raincoats and dandruff.