Their UI, or the service itself, is also flaky. It's fairly common for us to start watching a series and the next day Netflix will show us as having already watched all of the episodes, despite our changing to a complex password and also not enough hours passing for us to have watched them all. I wonder if they are juicing the numbers for some reason or if it's just an error.
I suppose it's possible that their auto-play feature is doing it. Most of our Netflix use is on a laptop so if the browser tab isn't closed maybe it keeps streaming the auto-play, although I haven't checked for that.
