Some web servers, such as that produced by Netscape, and Microsoft, are not compatible with PageInspector's Internet plug-in.

If PageInspector reports during the checking a web-based link that the server responded with a message that PageInspector does not support (such as a server message 500), there is a good chance that you are connecting to one of the aforementioned web servers.

When checking links, instead of downloading the link (which may be quite large), PageInspector requests that the server send only the details of the link.  This is what Netscape and Microsoft's web servers do not like, despite it being part of the standard HTTP protocol.  There are no plans to fix this "bug" in PageInspector.