SharePoint limits URL to 260 characters
SharePoint is a Web content management system. Within SharePoint, you can create document libraries, directories, files. You end up having resources like http://my.sharepoint.site.localdomain/Public%20Documents/Director/My%20file.doc
If a directory or file is created, and the total length exceed this, you get a very inexplicit « Unkown error » message. Of course, I think this is bad behaviour: SharePoint should at least say the URL is too long.
The RFC 2616 which defines HTTP/1.1 even says this should be an error 414:
The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle