Keep in mind that users don't read

I was just updating my cell phone plan on T-Mobile's web site. They asked me to enter the last four digits of my social security number, or my birth date to confirm my identity.

I was just making a quick change, so I didn't read the instructions on the form written in font size xx-small. So I entered both the last 4 digits of my ssn, and my birthday. This caused the form to send an error back saying enter the last four digits of your social security number, or your birthday. Again I missed the OR, and after one more erroneous attempt I finally read it carefully, I entered just one, and it worked.

But my question of the day is, why would you throw an error message for providing too much information?

Also some advice, if you have a form with an option like that, it may be better off to add some more separation. I think perhaps having one option on the left, a big, bold OR in the middle, and one on the right would work better.