Revealing mistake: When "Mac" is told by the captain of St. George's to destruct the ATAC system, an explosion occurs and he gets thrown to the ground. When this happens, you can see that the man playing Mac is his stunt double.
Revealing mistake: Many of the underwater close up shots are done with a fish tank in front of the camera and bubbles superimposed where the actors' breathing apparatus should be, but the location of the bubbles isn't always correct. This is obvious in the first underwater diving scene at the archaeological dig with Bond and Melina. In some shots the bubbles appear to come from her hand instead of her regulator. The position of her hand matches the location of the regulator in close ups of Bond. (01:21:10)
Revealing mistake: When Bond enters Q branch, he is met by a technician with a false broken right arm. After saying "Coming along very nicely sir" the arm then swings to the technicians' right breaking the head of the dummy. This then reveals the arm of the technician, who then holds his hands suggesting the false arm is a 'Spy Gadget'. You can see that the revealed hand is not that of the actor but of someone stood behind the set wall. The actor has used his real arm to carry out the effect, and then clasped the hand of the extra person. This is supported by the fact the arm movement can be seen through the shirt, the skin complexion of the hand is completely different to the actor, the arm looks like he is being hugged, when he puts his hands together it is not natural, and as they move away he moves his right shoulder up and the 'third' arm remains in place.
Revealing mistake: When Apostis kicks Bond in the face when he reaches the top of the cliff, you can see it is not Roger Moore but his stunt double.
Revealing mistake: When Bond attacks the ice hockey player in yellow with his stick during the fight at the hockey rink, you can see the player reacts to being hit just before the stick makes contact.
Revealing mistake: When Bond jumps out of the way from Claus's buggy on the beach, it is not Roger Moore but his stunt double.
Revealing mistake: When Locque kills Countess Lisl by running her over in his dune buggy, look in the cockpit and you can see that the driver is not Michael Gothard but his stunt double.
Revealing mistake: Kristatos wants to run over Bond and Melina by boat, after he took the Atac. Bond and the girl swim to the sea floor, waiting till Kristatos is gone. You can see the (shadow of) the anchor hanging on the left side of the ship. But in the previous scene, you will notice the anchor is in the middle, on deck! And second: these kind of (speed)boats do not have an anchor attached below the water line, because it will reduce the speed. Conclusion: the underwater scene is fake.
Answer: The producers of the film had a falling out with Kevin McClory, who had the rights to the character of Blofeld. They decided to kill off Blofeld to show McClory that the films could manage fine without the character. Obviously, without McClory's permission they couldn't go so far as to say that the guy in the wheelchair is actually Blofeld, otherwise they'd get into legal bother, so they just left it to fans to assume that's who it is.
Madstunts