Well, I'll be careful now...but there was no way to know in advance of that kind of behavior.
Sure there was. On that turn 12 you took, where it says "toaster plays a Copper, a Gold, and a Silver," I'm guessing the button you clicked to make it do that said "+$6". The treasures that do something when played (Loan, Venture, Contraband, Bank, Horn of Plenty, Ill-Gotten Gains) are never played with the +$ button, so you could have compared the +$6 that the button showed to the Treasures you had in hand to figure it would play Copper, Silver, Gold. Of course, if this was the first time you had played with Loan you wouldn't have known that, and then you definitely would have been in the dark.
FWIW I'm pretty sure if you had played Loan first the button would have changed to +$5. It seems like the button doesn't really consider special Treasures before they're played. I agree it is an annoying bug that can lead to mistakenly playing Copper when you wish to buy Grand Market, but speaking from personal experience the rage-inducing accidental Copper play happened much more frequently before dougz added the feature. I guess it can be a nasty trap in these cases where you think you're safe but you're really not.
Anyway, here's hoping whoever's developing the hopefully-soon-to-be-released commercial version have followed discussions like this to some extent so they can do things like this in the best way possible.