Check your Apache error logs for recursion errors. I had this issue on two different servers and it turned out that every browser other than IE puts the slash at the end of the URL when it autocompletes.
It turned out to be my anti-hotlinking code. I had to replace:
Code:
RewriteCond %{HTTP_REFERER} !^$ [NC]
RewriteCond %{HTTP_REFERER} !^\-$ [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain\.(com|net|org)/ [NC]
with
Code:
RewriteCond %{HTTP_REFERER} !^$ [NC]
RewriteCond %{HTTP_REFERER} !^\-$ [NC]
RewriteCond %{HTTP_REFERER} !^http://(.*\.)?mydomain\.(com|net|org)/ [NC]
Note the third line where www was replaced by .* Also be sure you have the trailing slash in the rewrite for your site. The one after (com|net|org) above. It has been awhile and I don't recally *why* that fixed it.
If you aren't doing anything similar to the above it's still likely mod_rewrite going nuts. Turn it off for a moment to see if you still have the problem.
Hope that helps.