#Character at the end of the url

I'm not sure how to word this question (or the subject), but hopefully this will do:
I have an ASP.NET 2.0 web application with asp.net web controls that must be 508 compliant.
The problem is when any character is typed into a <asp:TextBox> (multilined or not) or even an html <textarea>, the character that is typed in is displayed at the end of the url preceded by a "#" sign. This is causing my application to fail 508 testing. My code is:

<asp:TextBox ID="TextBox1" ToolTip="My TextBox1"< ...