Happened to me after thinking for some while about my buys and deciding that I don't need to use coin tokens.
The 0 coins position on that slider is a strange beast. The game should not need it (if I do not want to spend coins, I simply should not need to interact with the slider). OTOH, in some rare situations the 'end buys' button is not there, and moving the coin slider to 0 coins and pressing the button is the only known way to end the buy phase without spending coin tokens.
This is certainly an interface bug, but at the moment it is a mitigation that helps in another bug scenario. Let's hope those bugs get fixed in the right order.
I might not fully understand the backend, but I think the case where you need to use 0 is such that the game engine waits for your decision. That would still be achieved if clicking 0 doesn't end your buy phase. The buy phase then ends because you can't buy anything.
Yes I should solve this.
The problem here is twofold:
a) you are are in a situation where the only thing you're allowed to do is spend coin tokens.
b) you probably don't realize this. It's a silly situation to be in, off course you don't want to spend your tokens for nothing.
The gameserver now just asks "ok how many are you going to spend?" e.g. [0-4].
It would be much more intuitive if I would modify that question: present the choice as [1-4] and add the appropriate button ("end turn" or "end buy phase")
You would still wonder why you'd have to click at all, but at least you would know what to do and what the consequences are when you do it.
An interesting question is if we can skip this question altogether. With the recent possession ban maybe the only edge-case where you'd need it is removed. But you can still handpick possession so we can't just go ignore its existence. I could maybe skip the question on all boards without possession or black market?
If possible, make the decision: "Board does not contain Procession and does not contain Black Market and Potion". This should further reduce the number of games where the game has to ask.