I have been finding that users are managing to register on a site without a Date of Birth, despite having it set as required for registration. After a bit of testing, this happens when they enter something that is not a valid date into the control. On saving, the value disappears, though everything else saves OK.
I've improved this with a regex validator, requiring something that resembles a UK date. However, bad ones can still get through, such as 31/02/2014 (etc etc) which are not easy to tackle with a regex. I'm rusty on ASP.Net validators, but could there be something extra on this field to ensure that a valid date has been entered? Maybe a "compare" validator?