Why not pre-empt it and tell your new boss - you don't have to say that you knew at the time it was wrong, but something along the lines of: I realise that the directions I was given by X on this piece of work were incorrect, I apologise that I didn't pick it up at the time (stress of sick father, they were the instructions I was given etc.). It can be fixed by doing Y - would you like me to rectify it?

That way you are being proactive, admitting that you should have spotted it at the time but that the initial instructions given to you were wrong, and that you have considered how to fix it.