Very odd!
Yes, our Jack once went really weird. It was one evening, he was in his stable, he was wild eyed and went to the back of his box as though he was terrified if you went into the stable with him, but not necessarily terrified of the person - I could go in and touch him - but it felt quite dangerous because he was so obviously wired. I rang the vet, it was so strange I thought he must have a brain tumour or something. The vet just said to keep an eye on him and he'd be out the next day if there was no change but by next morning he was back to normal as if nothing had happened. If something had happened to spook him we have no idea what it was. A few years later a similar thing happened with one of YO's horses, but she was a young Section D (prone to dramatics) mare who hadn't been on the yard long, plus it wasn't quite so extreme so not as worrying. She did it a couple of times but hasn't done it recently.
Oh, and once a long time ago I was hacking the horse I used to loan down a normal route and he kept looking over the fields to the left and snorting. He did it for most of the ride - there was nothing I could see out of the ordinary but a horse on the yard had just been PTS the day before and I ended up finding it quite spooky