The worst number input that I've seen is some Windows input, in which the value must always be valid and in the allowed range. So, say, the current value is ‘300’ and you want ‘100’—but you can't backspace over the ‘3’ and enter ‘1’. If the value is ‘350’ and you want ’100’, you can't even select ’35’ and type ‘10’, because the input will freak out midway and immediately replace the value with the allowed minimum. Moreover, how do you even type the number you want if the input can't be empty? If the minimum is 20, you can't ever type anything from scratch without doing some select-replace gymnastics, because every digit will be immediately replaced with the value 20.

Can't remember which app it was, and if it was a standard input from one of the dozen MS frameworks—but the testers on it must've been mightily incompetent. Imagining for a second that someone worked on that is like catching a glimpse into an abyss of pure stupidity. After this horror I'm quite relaxed about problems with inputs that at least allow me to type the damn number.