Revealing mistake: In the scene where Chitty is taking off from the castle, Caracticus looks down at the toymaker and Peter and says, "Bye bye". If you look closely, right in front of them, a boy runs across holding a stick he stole from one of the cavalrymen. He is waving it around and you can tell it's fake because it flops around like rubber. (02:16:50)
Revealing mistake: When Walther Matthau is making sandwiches for his poker pals (during the movie's first poker game), Matthau drops a slice of bread on the kitchen floor. If you look at his hand, you'll see that he flicks his thumb to make the slice of bread fall.
Revealing mistake: Whilst imprisoned by the Burpers, Brother Belcher calls to one of the guards. The guard's supposed to have many missing teeth, but they have just been very obviously blacked to simulate this (a very common film or theatre trick, but usually not this obvious).
Revealing mistake: When Carole and Jim rush out of Herbie to find Thorndyke under the bonnet, you can see that Carole is not Michele Lee but her stunt double.
Revealing mistake: When Philip is in the bathroom at the new house Mike puts toothpaste on his toothbrush and then Philip smiles to reveal missing teeth. Look closely at Philip's teeth and you will see that they are not missing at all - there is black paint on his teeth to make it look like they are missing. (01:14:40)
Revealing mistake: When Matt and Freya are attacking the compound, at one point he goes outside and throws a "hanky panky" grenade at what looks like a garage door. He throws himself over a hedge and a moment later the grenade explodes. When he is getting up, the "ground" is obviously a cushion covered with fake grass.