0.1

0.1, zero point one, and any numbers non integer is very annoying when you program. Because all the information in the computer consists with 1 and 0. And all the numbers are expressed with 1, 2, 4, 8 and like. So when you express the number, it is approximated value. As a result, the very simple summation calculation doesn't completed; 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 doesn't equal to 1.
Very bothering phenomena in the computer under the "point".