The OP may not have made an error; many internet sites use scroll bars for selecting data entries, these sometimes "move" when you move your mouse anywhere on the screen--to prevent this from happening, in many instances, you have to click the mouse over an empty area.
On AA.COM for example, when I check in for an intl flt I have to enter my birthdate and expiration date of my pp, the scroll bars for the month and date(s) have a way of scrolling off what I originally selected.