I awoke to 1500 exceptions published to me this morning from our public site running ASP.NET 2.0. Not good times. "Luckily" they were all the same: Value cannot be null. It seems one of the components that we are using is calling Request.Browser.MajorVersion and blowing up on goofy user-agents. In this case the user-agent of doom is "YahooSeeker/1.2 (compatible; Mozilla 4.0; MSIE 5.5; yahooseeker at yahoo-inc dot com ; http://help.yahoo.com/help/us/shop/merchant/)". For whatever reason ASP.NET 2.0 cannot parse the MajorVersion out of that and tries to call int.Parse on a null value, causing an error.
Luckily we own the source code to this particular component, so I just wrapped the calls to MajorVersion in a try block so that the entire site doesn't blow up on bogus error codes, so hopefully if you run into that particular error you'll be able to track it down. I haven't tried any of the other properties of the Browser class, but if you are relying on them for anything you might want to first test them out with a rediculous user agent and see what blows up.