The directory listing format of FTP servers
One of the big problems with RFC-959, is that it does not specify a format for the output of the LIST command. Each FTP server is free to list the files in any way it want - and a FTP client must have support for each server-type in order to interperate the listing.

Old mainframe and minimachine servers can use really odd formats. Most new servers will however use the format of the UNIX "/bin/ls -l" command. The exact output can vary a bit from server to server.
The Microsoft NT service use a format similar to the output of the MS-DOS "DIR" command.

If you write a FTP client, you will have to do some research in order to support all common FTP servers :-)

An other problem is that some servers accept unix "/bin/ls" style command line options (like -Rla") while others will try to interperate any characters following the LIST command as a pathname.

These problems are addressed by the FTP Ext Working Group and will be resolved in a future RFC.