Default Input Validation In ASP.NET

Working with a Context.Handler I tried to input HTML comments from page1 to page2. This was before I added any validation controls whatsoeve.Didn't expect the following run-time error:

A potentially dangerous Request.Form value was detected from the client (txtValue="<b>bla!</b>").
Description: Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. This value may indicate an attempt to compromise the security of your application, such as a cross-site scripting attack. You can disable request validation by setting validateRequest=false in the Page directive or in the configuration section. However, it is strongly recommended that your application explicitly check all inputs in this case.

A nice way to prevent malicious input just in case you forgot (eh, ..?) to do any validation. Must be a no-brainer to you but for me this was the first time I noticed this....