I don't know if this is a bug or a feature but I will report it as a bug..

When using ftp you are presented with a login prompt and eventually
a password prompt.  If you are successful in logging in the transfer mode
defaults to binary.

If the login fails and you succeed on a subsequent try the transfer mode
defaults to ascii.  This can be very annoying if you forget this happens
because you may transfer an entire rather large file before you realize
it's corrupted.