First dimensioning error. Francis, in my absence, decided to move a window by 5 cm (2 inches) to avoid having to cut a monomur. Working from the inside, he didn't see that it should have lined up with the window below. I made a fuss, but decided in the interests of good industrial relations, to not demand that he take down 2 courses and rebuild it. Margaret could not see the error - only an architect would probably notice it.