Continuity mistake: Kevin's, Stuart's, and Bob's goggles change from brass to steel between when we last see the goggles in the rowboat to when they climb ashore in New York. (00:11:40)
Factual error: When Kevin, Stuart and Bob are honored at the end of the movie in front of Buckingham Palace, the Union Jack can be seen flying on top of it. The only flag displayed on Buckingham Palace at that time was the Royal Standard, and only when the regent was in residence.
Chosen answer: The writers wanted to keep Gru's cameo at the end of the film a secret for the fans of the franchise. So, saying "42 Years B.G" (before Gru) in the trailer doesn't give the surprise away.