What you probably saw were errors like the following:

GSSAPI error major: No error
GSSAPI error minor: No error

There is an error here, it's just not being reported very well.

This comes from the ftp daemon; it is trying to build a service principal
for the ftp service using the fully-qualified local hostname as the
principal instance, and it's not finding it in the local keytab. This
generally results in a confusion between the machine's local hostname and
it's hostname in DNS.

Note that ftpd is currently the only program that deals with service
principals this way, so this is why these problems crop up with the Kerberos
ftp daemon and not other Kerberized services.