On the way to work in the subway, there is about one minute in the trajectory where my phone loses signal.
I happened to finish a review (of 19 items if I remember correctly) during that period of time. After that, this popped up:
After a few minutes, after I regained signal, I was returned to the deck selection screen, saying I had 7 items left to review. After I pressed review again, it instantly told me there weren’t any more items to review (and the counter was reduced to zero).