When I drafted vintage cube, I got an error that my deck was invalid. Some cards in my pool had alternative arts to the art used in vintage cube, for example, Thalia and Lurrus. These cards were both in my mainboard and in my sideboard with the default vintage cube art.

I suspect that the deck creator was double counting these, which threw the error. To fix the bug, I unhearted the arts of the duplicated cards and when I returned to the deck builder, the duplicated cards were removed and I was able to validate my deck.