This wouldn't have affected the outcome of the game I was in, but I just played a game against JoeDonahue in which
I ended a turn with 17 coin and 2 Buys (including +1 from Forum). There was one Province left, plus Duchies and Silk Roads, so I could have bought a second Victory card; however, as soon as I bought the last Province, "The game has ended" button appeared and I wasn't allowed to take my second Buy.
It is true that I would have lost no matter what else happened, as I was too far behind anyway; I thought I actually had three Buys, such that Province+Duchy+Silk Road might have allowed me to squeak out a tie, but this wasn't true. Nonetheless, I don't see why I shouldn't have been allowed to take my last Buy anyway. I don't see this in the list of known bugs, so I thought I'd mention it. Thanks for taking the time to read this and, perhaps, to investigate!
Daniel
Game number is 9892874.
You only had 1 buy. Note that Forum gives you +buy when you buy it, not when you play it (as you did on your last turn).