I like this saying because it encourages that mistakes are ok to make. I expect my students to show effort in the work that they do, and mistakes are fixable and acceptable as long as they keep trying. I would put this saying on a poster (on vinyl sticker like the one here) and keep in my classroom to remind students that it is ok to make mistakes.
Cute idea... good way to get them to learn square roots without them knowing it because they know the numbers under the post its! It is very inexpensive and not at all time consuming to make. The only problem I can think of with this would be the postit notes loosing their ability to stick onto the clock's face.