Earlier quoted context omitted.
4. Time-traveling humans from the future. Don't discount that possibility!
I do. If it were possible we'd surely see them everywhere. Even if time travel was a one-way trip there's enough future billions of us that there'd be massive numbers with the sort of incurable fascination seth the past that they'd be motivated to travel back and see what it was like. Doesn't really seem any more or less likely than alien intelligence at any rate.
It was about the first time travel trip. The team that developed the first time machine decided to send the first traveler to visit Shakespeare, figuring that Shakespeare had a flexible enough mind to not be freaked out by the visit.
When the traveler got to Shakespeare they were right that he did not freak out. In fact he took it entirely in stride. The time traveler was a little confused that Shakespeare was taking it so well. Shakespeare even asked what gift the time traveler had brought, saying that "all the early ones brought gifts".
The time traveler had in fact brought a gift--a nicely bound volume of Shakespeare's collected works. Shakespeare looked at it, said something about maybe he could sell the binding, then said probably not, and tossed it on a pile of books, which the traveler realized was a pile of similar books.
Shakespeare noticed that the traveler was now throughly confused and realized that the traveler was in fact one of the very earliest, and explained that most of early travelers brought books.
The traveler was still confused over the idea that Shakespeare had met other time travelers, saying "but I'm the first time traveler!". Shakespeare told him that he may have been the first to leave, but he certainly wasn't the first to arrive, and said at some stages in his life he was being visiting frequently by time travelers, which was actually annoying--although not as annoying as it was for Jesus, who Shakespeare says another time traveler decided to introduce them once.
At that point numerous other time travelers started arriving. They were reporters from throughout the timeline popping in to try to get an interview with the first time traveler. The first time traveller is now close to completely losing it, and Shakespeare says he can handle it and steps in to act as a press agent for the first time traveler.
If backwards time travel turns out to possible my guess is that there will be some limitation that prevents scenarios like the one in that story from happening. My guess is either (1) the time machine will only be able to go back to when it was created (think of it like going back to a save point in a game), or (2) when a time machine goes back to some point in spacetime it creates some sort of exclusion zone in a region around that point that precludes any other time machine from arrive at a point in that exclusion zone.