A fuller featured 404 error page modeled from http://1976design.com/blog/error/
This is known problem and a fix will be rolled into the next release. In the meantime, read on:
Apache makes a note below the ErrorDocument documentation:
Microsoft Internet Explorer (MSIE) will by default ignore server-generated error messages when they are "too small" and substitute its own "friendly" error messages. The size threshold varies depending on the type of error, but in general, if you make your error document greater than 512 bytes, then MSIE will show the server-generated error rather than masking it. More information is available in Microsoft Knowledge Base article Q294807.
My best recommendation is to inflate the size of the error document. But some garbage text between two comment tags and see what that does.
This is also a known problem and I'm working on a way to combat it. In the mean time, your best bet would be to disable the quick error report part. There are several ways you could do it, but the easiest and cleanest is to just remove lines 395 to 403:Quick error report