Continuity mistake: In the scene where Tim buys his lunch in a Pret A Manger, the assistant charges him £4.24. When he time travels back, she charges him £6.23. (01:40:05 - 01:41:55)
Continuity mistake: When Mary tries different dresses and shows them to Tim, Tim has increasingly messier hair. At the end, when she tries again the first dress, though, he suddenly looks the same as in the beginning. Presumably the first and last sequences were filmed together, then the rest.