I have thought about doing that for a long time. The reason I never did it is because 1. I'm too lazy, and 2. that could actually really cause problems. I look at the off by one as a joke, but that would be so unnoticeable that people wouldn't turn it off and would end up losing points on tests.
Like for instance when I put my friends float on 0 (I know it's not called float but calling it float causes the least problems). He took a test and thought that all the problems conveniently came out even. I tried to tell him while he was taking it but he didn't understand. Luckily I told him afterwards that someone else did it and the teacher didn't take off. But the off by one would just make someone look incompetent.