I think I found (and solved!) an Internet Explorer version detection issue.

I found that it couldn't detect between different versions, if I set a rule for IE6 it also got triggered in IE8 and vice versa.

In extendedrules.php I changed three lines and it works now.  The original code was:

            if ($b == "ie" and MM_BROWSER_IE == true) return true;
            if ($b == "ie6" and MM_BROWSER_IE == true and MM_BROWSER_IE <= 6) return true;
            if ($b == "ie7" and MM_BROWSER_IE == true and MM_BROWSER_IE == 7) return true;
            if ($b == "ie8" and MM_BROWSER_IE == true and MM_BROWSER_IE == 8) return true;

I think it's supposed to be:

            if ($b == "ie" and MM_BROWSER_IE == true) return true;
            if ($b == "ie6" and MM_BROWSER_IE == true and MM_BROWSER_IE_VERSION <= 6) return true;
            if ($b == "ie7" and MM_BROWSER_IE == true and MM_BROWSER_IE_VERSION == 7) return true;
            if ($b == "ie8" and MM_BROWSER_IE == true and MM_BROWSER_IE_VERSION == 8) return true;

It's working for me anyway.