Game 4075, I bought the discount coupon, and no other artefact until the final round when I grabbed the Altar of gold. Using the UI, I spent gold until it showed that I would have 2 gold left over, enough to endorse a theory, which would also get me one of the publish bonus token things. However, once it took the gold I was left with only one gold left, which wasn't enough to do anything with.
The only thing I can think of that would cause this is that since the discount coupon gives 2 gold off the first artefact, the altar costs 1 and it put the second gold towards being transferred as points, but then when I confirm my action it realises there wasn't enough actual gold and took one of mine to make up for it.
Steps to reproduce:
Buy Discount coupon
Don't buy any other artifacts
Buy Altar of gold in final round
Spend some gold on the altar, and look at how much gold you will have remaining
Click confirm and your remaining gold is different
Hopefully that's enough information to make this bug reproducible!