I suppose it had to happen eventually, and the miracle is that I've been teaching for a year and a half before this happened to me. I'd be horrified if this had happened, say, a couple weeks after I'd originally started.
I'm talking about forgetting my class notes, not realizing it until five minutes before class starts, and having to run a two-hour lecture entirely from memory.
I'm one of those people who likes to plan stuff in advance. I'm more comfortable when speaking to a crowd if I've already written down and rehearsed what I'm about to say. I'm much more comfortable if I have my notes in front of me, in case I get lost (which I often do). From talking with other teachers, this is something of a rite of passage. (Some day, perhaps, I'll take this to the next level: showing up for class without having prepared a lecture in advance at all, and just saying whatever occurs to me at the time. But that day will not come any time soon if I can help it -- I was never all that great at improv.)
I realize that not all teachers are like this. Some don't do any planning in advance at any rate, so this is the norm. Others rehearse things so many times that they've got it memorized, so they don't need any notes. I'm somewhere between the extremes, and for me it was really scary.
The thing is, I survived. And I'm not even sure my students noticed. I suppose it helps that I've taught this particular class three times already, so I had a pretty good idea of the general topics. After I got home, I reviewed my notes and found that I covered most of the major points, and I mentioned the rest in the following class as an aside. So, no harm done.
But I'm not going to try it again any time soon, all the same.