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: 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.