I managed to get this one to happen on stream. Here's a clip of it, but I'll also describe what happened here.https://www.twitch.tv/videos/500808512
Here's what happened. I played a rated automatch game against a person. When the game was over, I clicked "Ready" because I was OK with rematching this person, but that person left the table. My status was displayed as "Not Ready" when this happened.
Then, a spectator sat down at the table, clicked "Ready" and the game immediately started even though I didn't click anything. My status showed "Not Ready" during all of this, but once my new opponent clicked "Ready" the game started.
I should have to click "Ready" again in order to be ready when anything about the table changes, but that didn't happen here.