Sometimes when running your code, if you have a lot of validations going on, the page.isvalid might return a false. Leaving you scratching your head, trying to figure out which of the 30 validators on your page, is the one firing.
Here is a piece of code to try on the button click event or any other event for that matter.

using System.Linq;

List isvalidErrors = this.Validators.Cast().Where(v => !v.IsValid).ToList();