What would happen if one time traveler traveled back to a time that interfered with the acts of another time traveler?
We never see this in the film, yet it must have happened many times--we know that Dad lived every day of his life twice, spending it first reading and then returning to live his regular life. Tim also made trips to the past, some of them reaching back several days. It would seem that sometimes what one of them does must interfere with what the other does. But let's tackle an obvious example, from the wedding.
We know that they put the wedding plans together quickly, because Mary was already pregnant; Tim chose Rory to be the Best Man. The toast was a boring disaster, so sometime after the toast but presumably before the end of the reception Tim slipped away from the party and went back probably to the night when he decided on Rory, and changed the decision to announce that Harry would do the honors. Harry was rude and crass, so again Tim makes the trip and chooses Jay. Jay decides it's time to be ribald and embarrass everyone, so there is one more trip, and Dad is now the Best Man giving the toast.
We do not know when Tim makes the change, but the impression we have is that he does so at his first opportunity following the toast. Thus when we see Tim and Dad talking at the reception, when Dad decides he can do better, we have good reason to think that this is at a moment after the last trip Tim made to make his father the Best Man. Yet when Dad travels back to change the toast, that trip takes him to a moment before Tim made that trip, and rewrites all the history thereafter.
This is the same problem we addressed when Tim attempted to prevent himself from taking Kit to the past, but with a new wrinkle: if history changes such that the time traveler never departs from the future, does that undo his arrival in the past? This instance must be particularly confusing for Tim, though. Forward to the reception following the second version of Dad's toast, and Tim will remember Rory's toast and the trip to change it to Harry's toast, and the trip to change it to Jay's toast--but when Dad erases his first toast and replaces it with another, Dad also erases Tim's memory of the first toast and of the decision to replace Jay with Dad. He will remember making the first two changes, and he will remember that Dad wound up being his best man (when it came to the toast--what he remembers of events before that we do not know) but not how it happened that Jay was not the one who delivered the toast.
Or will he? We again have the problem of what the time traveler remembers, only this time when another time traveler is involved. That Tim remembers all versions of the toast suggests that he went back far enough to change the best man and then came forward only as far as the time of the toast, so he could hear the altered version also. We have seen that neither Kit nor Tim remember events in altered histories they never experienced. When Dad travels back to redo his toast, he must erase the history in which Tim made the trip to appoint Dad to do the toast--and while we have been generally accepting the idea that once a history has been changed it stays changed, that still leaves Tim wondering how it was that his father delivered the toast when his memory is that Jay was to have done it.
In fact, now Tim should not remember Jay's toast; possibly Tim would not remember any of the toasts except perhaps Rory's and Dad's, because his father has erased every history in which Tim changed who gave the toast.
No answer seems to make sense here; the film, wonderfully warm and enjoyable, does not withstand scrutiny well.
We have one more article ahead.