Deliberate mistake: Michael B. Jordan has a small indentation on his forehead, just above his right eyebrow. By virtue of the fact that he plays twins in this film, both characters have the exact same indentation.
Deliberate mistake: Concerning the literal "bag drop" off the subway car: The ransom money bag falls from the train directly into the waiting hands of the bad guy, after it falls very arbitrarily from Denzel's hands, after the train is brought very arbitrarily to a screeching halt by another bad guy very arbitrarily hitting the emergency stop. This was sheer fantasy that could never happen in real life even if the thousand unknowns were known. Bigger than a simple plot hole - this was a deliberate mistake.
Deliberate mistake: The film strongly implies (especially given the children barely age at all) that the events depicted happened over a period of only a couple of years. In fact, Mike and Claire met in 1987 and married in 1994; Claire lost her leg in 1999, and Mike died in 2006.