When using a number of Validation Controls on a page, I was encountering a bug in which the form would submit even with invalid data.
I had RequiredFieldValidator Controls for a number of inputs in a form, and also a RegularExpressionValidator Control. Within this RegularExpressionValidator I had a rather large regular expression (the RFC 2822 standard Regex below);
With this regex in place, whenever I would enter bogus information into the email input on the form that had both this regularexpressionvalidator AND also a requiredfieldvalidator used on the email TextBox control, The rest of the form would be validate, even though it was either left blank or filled with invalid data. So this value appeared to cause the other validators to fail.
Once I changed the regex value to something smaller, such as this;
The form's validation behaviour returned to normal and the page would only validate with valid data.