Every now and again (or more often than that depending on the kinds of day you’re having) a mistake gets made. You put the X on the wrong side of the line, read a 6 as a 9 and put a mark that doesn’t signify anything, or your stud finder is — as most are — wildly undependable. The result is you’ve somewhere between 1 and 58,000 pencil lines that you need to get rid of.