Okay, I'm fairly annoyed.
I wrote up a very funny notebook document with beautiful TeX equations that outlined how to heat food in a microwave if your unit doesn't deliver the power stated in the cooking instructions, and while I was doing it, ObservableHQ logged me out.
It gave a warning about it, so I signed in again, and...
Boom.
Hours of work was reverted to the last save state on my account.
What an extraordinarily stupid way of designing software!