I think my favorite "feature" of the FTP protocol has to be ASCII mangling, wherein the FTP server tries to mess around with line endings and text encoding mid-transfer. It's so bad that vsftpd, one of the better FTP servers for Linux systems, pretends to support it but silently refuses to perform the translation.
I wrote a custom FTP server once (it was database-backed instead of filesystem-backed - e.g. you could do searches by creating a directory in the Search directory) and I added in insulting error messages if a client tried to exercise one of the more antiquated features of the spec (e.g. EBCDIC mode)
http://sdocs.readthedocs.io/en/master/sdocs/ftpserver/vsftpd...