Some descriptions of this error I've found around the 'net are:
Components of path require hopping to multiple remote machines and file system type does not allow it.
This error occurs when users try to access remote resources that are not directly accessible.
I do not know which OS's this is used on, or what they are talking about. It's not used in the Linux kernel, and POSIX sheds it's usual level of enlightenment with it's riviting description of "Reserved".
Actually this is used for AT&T Remote File Sharing, which is an old 1980s alternative to NFS. See 
http://en.wikipedia.org/wiki/Remote_File_Sharing - so operating systems such as AT&T SVR3, AT&T SVR4 and Sun Solaris 2.1 could produce this error. In more recent OS, this error never happens, but the error code has not been removed due to backward compatibility. I believe it is caused when machine A exports it filesystem to machine B, and then machine B exports its file system to machine C - machine C is not allowed to access machine A via machine B, so when C asks B for a file which B gets from A, B will return EMULTIHOP to C.
