Update…I did that a couple of days ago. It felt as though I blinked. I was lying in bed and looking at the window. It was completely dark, around 10. Then I blinked and it was day, 8 in the morning…weird
That’s happened to me countless times, I hate that so much. 
When I did my two week experiment with the dimaxion sleep schedule I noticed this all the time.
My half hour sleeps would pretty much always follow the same pattern. I would fall asleep for 15 minutes, and wake up thinking I had overslept by 10 or 20 minutes, check my clock and realise that I had, in fact, underslept. I would then fall asleep again for a further 15 minutes and go through the same process.
I’ve had this quite a lot too. Whenever I start a new semester and haven’t got my sleeping patterns sorted out, I inevitably start drifting off in lectures and having the exact same thing you just described. It’s even weirder when I’m also partly listening to the lecturer but interpreting what they’re saying in terms of the ‘nonsense’ that’s going through my mind.