Or, what I learned this weekend.
Had a little 'outside' project to build; a small, sort of triangular frame with a 3/4" ply skin. (Think 45-right-triangle, with a couple of corners nibbled off).
Took my notepad outside, measured, and re-measured; drew, and re- measured some more. Set up a couple of sawhorses, and layed the cuts out on the plywood. -- Cut the piece out, and took it to its new home.
Did I mention that the plywood had one good face, and one not?
D'oh. Apparently, in the process of laying the cuts out on the plywood, I'd transposed left and right (or more likely, moved my reference from the "back" to the "front").
So the piece fit perfectly. With the good face *down*.
Well, rather than call it a mistake, I decided that I just needed a full sized template. I still had a half-sheet available, so I bit the bullet, and cut a new piece, this time making sure to get the good face *up*.
Oh well, it turned out ok in the end; and it was a nice afternoon to make sawdust. Just a little more than I'd planned.