Revealing mistake: The Union Flag pinned to the back of the taxi driver's jacket is upside down. Since he is part of the government (he's undercover for the Foreign Office) he shouldn't make such an terrible mistake. (01:20:40)
Revealing mistake: Mata is kidnapped and taken to Casino Royale in a flying saucer, which then releases a small hovercraft to bring her inside the island cave. Once inside the cave, the hovercraft leaves the water and goes up a ramp, at which point a tow cable can be seen pulling the hovercraft up the ramp and onto the landing platform. (01:45:25)
Revealing mistake: When the UFO lands and the ramp is deployed on the road, the last part briefly turns transparent. Additionally, a man walking past a red car fades and disappears as he approaches the ramp - a clear result of the composite image effect. (01:47:52)
Revealing mistake: During the saloon fight, when a cowboy blows his trumpet, the hole that the arrow later passes through is already visible.
Revealing mistake: When the strongman grabs the boulder, falls backwards, and it breaks through the floor tile, a clear rectangular pre-cut in the floor is visible - made to facilitate the stunt.
Revealing mistake: During the casino fight, when Bond kicks a man, he is suddenly replaced by an obvious stunt double with much thicker hair.
Revealing mistake: When the UFO releases the hovercraft, vertical creases are visible on the blue-sky backdrop behind it.
Revealing mistake: When the crowd watches the UFO, the group of people on the right can be seen through the now translucent tramp.
Answer: Given that this is a comedy, the thinking was probably "Well, we'll just blow up your retirement so you've got no choice but to come out of it."
Captain Defenestrator