True, and what would he have done anyway. Put 2x18 in parentheses and add the +1 after the fact.

I had a professor who once made a simple mistake while solving an equation and he had a doctorate in math which he damn well deserved. It was a curious thing since his eventual solution to the equation he was working on was correct. Anyway I pointed out his mistake since I seem to have been the only one to notice it and he simply looked at the board and then corrected his mistake. As it turns out he had already solved the equation in his head, thus the correct solution, and was simply not paying attention to what he was writing. This though, is definitely not the same.