FTP
503 Bad Sequence
Bad sequence of commands. The command is valid but was sent in the wrong order relative to other commands.
When You See This Code
When you skip a required step, like sending PASS before USER, or RNTO without first sending RNFR.
How to Fix
Follow the correct command sequence. Common sequences: USER then PASS for login, RNFR then RNTO for rename, PASV then RETR for transfer.