I've noticed a curious phenomena popping up in my error logs recently. If, as the result of processing a form, I redirect my users to the URL http://www.example.com/index.php?foo=bar&bar=baz, I will see the following two URLs in my log

http://www.example.com/index.php?foo=barbar=baz
http://www.example.com/index.php?foo=bar&bar=baz

The first one is obviously incorrect and will cause my application to redirect to a 404. It always appears first, usually a second before the second one. The 404 page is not doing the redirection, so it appears that the browser is trying both versions. At first, looking at my server logs made me believe it affected only Firefox 3.6.3, but I've found an example of Safari being afflicted as well. It happens fairly intermittently, though it can occur multiple times in a users' session. I've never been able to get it to happen to me.

Any thoughts as to the nature of the problem or a solution?