At 1:20:37 an escaping Slater, whose hands have been closely cuffed, is for the moment free of the handcuffs, which are back on a moment later.
The teletype to Slater informing him of his son's death is addressed to him in "D C" (no periods). The one he throws on the desk before leaving reads "D.C." (with periods).
When Dan and Gina get on the first cable car, Col Berthold is shown at the very front of the car up against the window. Moments later as Dan moves up to talk to Gina, he is shown in the middle of the car watching Dan.
During the skiing sequences, the material used for the rear-screen projection is clearly visible - as if it were dirty.
The teletype to Dan Slater near the beginning of the film is addressed to him at the CIA, Washington, DC. CIA headquarters was and is actually in Langley, Virginia, and has been since 1961.
Slater's son is killed in a skiing accident and when Slater arrives in the ski village the next day the funeral is already in progress in the church. It is illogical that the funeral would be arranged so quickly and especially without the next of kin and family involvement or waiting for the family to attend.
After the substitution of Slater he is kept alive and transported in the car providing the opportunity to escape. Once the switch is made Slater would have been easier to dispose of as a dead body in the trunk.
After Slater escapes from the bad guys hideout he inexplicably isn't wearing his overcoat. There is no logical reason for him to return to the hideout later other than for the viewers to see the two Slaters together and know there has been a switch. When Wheatley and Miller come to his rescue Slater is just sitting there by himself in the dark waiting but now has his overcoat on. Slater had the school bus available and could have left anytime.
Gina is presumably questioned by the police and several times by Slater about Robert Slater who dies in the skiing accident and never clues in that Slater is the boy's father and has to be told much later by Wheatley.