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.
Thanks for the report.
It seems not to be a glitch, I could reproduce this behaviour in all tests. From the point of view of the player becoming your new opponent, it shows 'you' as ready, so it looks like your client showing you 'not ready' is actually the error, even though it is the state one would expect.
Well umm. When I click ready I have the expectation that the game will start exactly as I want. I don't want the terms to change and then be surprised to find i've started a rated game that I didn't consent to.
The client's behavior should definitely be changed. It's beyond me why anyone would want this behavior.