Factual error: When the minions are attempting to purchase tickets to get in to see/steal the crown, the admission prices on the wall are priced as £s and pence (decimal currency). As this was 1968, Britain was still using pre-decimal pounds, shillings and pence.
Factual error: American style fire hydrants are seen at least twice, but they aren't used in the UK.
Factual error: During the night chase scene through London streets, a subterranean vehicle emerges from the ground. The drill head is turning in the wrong direction and would be ineffective for the same reasons as turning a screw in the wrong direction.
Factual error: While hiding in the arctic cave, one of the minions eats a banana. However, the minions are supposed to have been trapped there for many years and edible bananas are restricted to tropical or near-tropical regions, roughly the area between latitudes 30°N and 30°S. It would not be possible for them to have bananas.
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.