It seems that the friendly URL handling removes multiple values from querystring parameters. I have a URL like this:
http:/the.site.se/the/friendly/url/?item=1&item=2&item=3&submit=Compare
but Request.QueryString.GetValues("item") only has one item (the last one in the URL, ie 3). And with Trace turned on the querystring is reported as
id=21&item=3=submit=Compare
It would seem that this is a problem with the friendly URL rewriting. Changing the form to use a POST request fixes the problem, but I don't really think a POST request is appropriate.
It also seems that friendly URLs cut off parameter values at the first ";" or "#" character (and possibly other characters), even if these are URL encoded according to the URL shown in the browser. Obviously, query strings must be used with great care and contain values only in very well-defined formats.