Days of Future Past: Part 2 - S1-E12
Factual error: The U.S. flag has 13 stripes. When Bishop is telling the X-Men how Gambit's betrayal will affect the future, the flag shown flying outside the Capitol building is missing two stripes. In the next shot showing the flag draped over the coffin, it has at least 17 stripes. (00:04:25)