When ssh, if you get:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!

Someone could be eavesdropping on you right now (man-in-the-middle attack)!

It is also possible that a host key has just been changed.

The fingerprint for the DSA key sent by the remote host is

<<<<<blah >>>>>

Please contact your system administrator.

Add correct host key in /Users/<username>/.ssh/known_hosts to get rid of this message.

Offending DSA key in /Users/<username>/.ssh/known_hosts:20

DSA host key for <ip address> has changed and you have requested strict checking.

Host key verification failed.

Just run:

ssh-keygen -R <hostname or ip address it complained about>