I got this dangerous error while using the Topics Form and the Reply Form.

A potentially dangerous Request.Form value was detected from the client (Message=" testubg").

The above error occurs if html code is placed on either the topic form or the reply form. For example:

<b> will produce the above mentioned error (VB.net version of thephilebv)

check this out:
http://www.asp.net/faq/RequestValidation.aspx

Thanks, I was about to lose it...