My senior undergrad year, Princeton offered their every-three-years-or-so undergrad general relativity course (AST 301), taught at the time by J. R. Gott III. Prof. Gott ran a pretty fun class, and he was a droll lecturer with a trace Southern accent and a dry sense of humor. He was most well known at the time for solving the equations of general relativity for the case of cosmic strings, sort of 1d analogs of black holes. He'd shown that if you have one cosmic string move past another at speeds approaching the speed of light, you could in principle go back in time.
The lectures were in a small tiered auditorium with the main door in the front, and a back entrance behind the last row. On one Thursday in the middle of the semester, we were sitting there waiting for class to start, when the front door of the auditorium flies open, and in bursts Gott, with (uncharacteristically) messy hair and dressed (unusually) in some kind of t-shirt. He dashed in, ran over to the utility closet in the front of the room, tore it open, and threw in a satchel of some kind before slamming the door. He turned, wild-eyed, to the class, and proclaimed, "Don't be alarmed by anything you may see here today!" before running out the front door.
This was odd. We looked around at each other, rather mystified.
Two minutes later, right at the official start time for class, the back door of the classroom opened, and in stepped a calm, combed Prof. Gott, wearing a dress shirt, tie, and jacket. He announced, "I'm really sorry about this, but my NASA program officer is here today on short notice, and I have to meet with him about my grant. Don't worry, though. I've arranged a substitute lecturer for today, who should be here any minute. I'll see you next Tuesday." He then left by the back door.
Another minute or two goes by. The front door opens again, and in steps a reasonably composed Prof. Gott, again wearing the t-shirt. "Good morning everyone. I'm your substitute lecturer today. I've come back in time from after next Tuesday's lecture to give this class."
This was met with good-natured laughter by the students.
"Unfortunately," he continued, "I didn't have time to prepare any transparencies for today. That's fine, though, because I'll just make them after the lecture, and then go back in time to before the lecture, and leave them somewhere for myself. Ahh - I know! The closet!" He walked over to the closet, opened the door, and retrieved the bag with the slides. There was more laughter and scattered clapping. "Of course," said Prof. Gott, "now that I have these, I don't have to make them, do I. I can just take these slides back to before the start of class." Pause for effect. "So, if I do that, then where did these come from?" More laughter.
Prof. Gott went on to deliver a class about time travel within general relativity (note to self: I need to read this book!).
Postscript: The following Tuesday, Prof. Gott arrived to teach class wearing the t-shirt outfit from the previous Thursday. We were suitably impressed by this attention to detail. As he walked by handing back our homework sets, I noticed that his wristwatch had a calendar on it, and I said that we should've checked that last time. He hesitated, smiled a little grin, and then went on.
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