I was made aware of the fact that curl is not really dealing well with the directory part of an ftp URL.
I was gonna quote the appropriate text piece from RFC1738 (yes, it is obsoleted by RFC2396 although 1738 has more detailed info about particular protocols like ftp) to someone when I noticed that I had interpreted it wrong when I read it before.
The difference between getting a file relative the login directory or with absolute path. It turns out you have to get a path like ftp.site.com/%2etmp/ if you want have the absolute path "/tmp". Oh well, I have it support my old way as well even if that isn't following the RFC just to allow people using that way to be able to use the new one unmodifed...
