When Tim and Mary leave Joanna's party, Mary is putting on a cardigan with only her right sleeve in the rear shot. But when the angle shifts to front, Mary has her left sleeve in already.
On Tim and Mary's "official" first date at the restaurant after the party, Tim's hair is sticking out on the left side of his head. The camera pans away with a view of the two sitting across the table from each other. When Tim is seen head-on again, his hair is not sticking out anymore.
Tim's father tells him that he is unable to travel back in time to before the birth of his children because doing anything slightly differently would result in a different sperm at a different moment, resulting in a different child. This might make it seem that Tim would be unable to travel back past the conception of his children, rather than their births,. But it's not about the science of the thing, but rather about Tim's lived experience: the child being born to and known by Tim.
When Tim goes back to the past to stop his sister Kit Kat from meeting Jimmy, he does it long before he met Mary, then when returning to the future, his life would be completely different from what he had, since in this new timeline, he would never have known Mary. But the "rules" state that only the birth of children prevents manipulation. Many, many things (big and small) are manipulated and changed throughout the movie.
Tim's dad tells him up front they cannot travel into the future. Yet on his first and subsequent time travels, he travels into the future from his present time.
However, this is not the future; it is a point in Tim's past. (i.e., it's already happened to him.) The point is that he cannot project himself to a point that has not yet occurred.
However, this is not the future; it is a point in Tim's past. (i.e., it's already happened to him.) The point is that he cannot project himself to a point that has not yet occurred.
After Tim and Kit Kat go back in time to fix her life, she knows all about her new life with Jay as soon as they return, but Tim doesn't know about his daughter Posy becoming a boy after he accidentally alters her existence.
At the end, Tim travels back in time to see his father, just before his third child is to be born. It is said that after the child is born, Tim will no longer be able to travel back to see his father without altering his child. Yet on this last journey, both Tim and his father travel back in time to when Tim was a boy. Tim's dad notes that they'll be "breaking the rules", but if they're "very careful and don't change anything, it should be all right." Their taking this risk does not change the basic principle of not going back before the children are born.