Last time we covered Tim's second and third first meetings with Mary, and the fact that having thought their "first time" was less than adequate he went back and redid it--twice. That raises another issue we saw in the Butterfly Effect films: what happens if a time traveler returns to the same moment in the past again, intending to do something different than he intended the previous time?
It is in some ways more complicated in this case, because we are relying on that uncertain interpretation of Niven's Law, that once he changes the past it remains changed. It is inherent in the process that an arrival in the past which changes the past erases the departure in the future that causes the change, but does not thereby eliminate the arrival in the past. When Tim learned of the disaster which was the play, he traveled back to prevent it, but then in the new history he did not have any reason to travel back to prevent it because it had been prevented. Where we would expect an infinity loop the film gives us a stable history, which it can only do by assuming that the changes made by the time traveler, once made, are not dependent on the time traveler departing from the future--he arrives in the past even though he does not depart from the future.
Of course, the problem is what happens if having arrived in the past and having altered the past, he then does depart from the future aiming for the same moment in the past--and more, if he does so intending to change a history he has already changed. Logically, his other time traveling self is already there. We have that problem with divergent dimension theory, in which the fact that the time traveler does not have to leave the future to arrive in the past leads inevitably to the conclusion that if he does leave from the future he duplicates himself in the past.
In this case, though, we have insufficient information. In The Butterfly Effect, Evan Treborn experienced blackouts, unable to remember the events of those times when his future self was in control of him (although as we suggested it makes more sense for them to be separate problems); the absence of such blackouts became serious problems for the sequels. In this case, we do not know what Tim experiences when his future self takes control of his body, or what he remembers of those events if his future self then returns to the future. Clearly Tim is erasing the departure and seizing control of his past self; that he then does so again and seizes control of himself from another self who had previously done so is strange but not more incredible.
He does this again when he once more encounters Charlotte. He comes close to doing so when he takes Kit back to the same party, but seems to leave before he arrives, so it probably is not a problem that time.
We will touch on one more little trip this time, because of the mistake it erases. Mary announces to Tim that her parents are visiting from America, soon enough that he needs to don some pants. She tells him that as far as they are concerned he does not live there, that if it comes up they are having sex but not a particular popular abberation. He jokes about how that could never arise, the parents enter, she almost immediately tells them that Tim actually lives there with her and he immediately adds that they are definitely not engaged in that particular practice. This he erases. I mention it because it is another trip to the past, with all the problems we have already identified, but also because it recognizes an aspect of human psychology that experts have only recently recognized: if you attempt to remember not to mention something, it increases the probability that you will mention it almost to a certainty. Thus the worst thing you can do before a conversation is tell someone "Don't say anything about..." unless you want to ensure that they mention it in some way. Tim, though, erases the blunder, and Mary tells them how much she loves him.
Next we return to Charlotte.