this is probably my computer since it didn't want to start the recorder, but this is a very strange thing i found.
I was playing survival, and I started to see missing chunks on the floor of my house. the sound wouldn't play. I placed down some glowing mushrooms then I left the world and went back in. it was fine. Eventually I decided to sleep, only to wake up to a zombie. I died and then minecraft ran out of memory. I ran the world back up, and I switched to creative mode and I flew up and I saw these... very large black grids. They seemed to act like NPCs. Some of them moved up and down, as if they were laughing. I didn't want to go near them. I paused the game to record, but it wouldn't work. I went back in, only to pause the game again. I tried to record one more time, and it worked. But minecraft crashed.
Everything seems normal now, this was a very weird experience.
EDIT: It happened again, this time I took screenshots instead