Sometimes when I run the ssh command I get this error:

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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
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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 the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx.
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending key in /home/user/.ssh/known_hosts:7
RSA host key for 192.168.1.5 has changed and you have requested strict checking.
Host key verification failed.

This usually happens when I change any machine in my network. The best way to solve this problem is removing all keys belonging to hostname from known_hosts file. In my case, the remote host is 192.168.1.5, so I run the command:

ssh-keygen -R 192.168.1.5

And now I can access again to the remote host.

An alternative is to set StrictHostKeyChecking to no in your ssh_config. For a better description, take a look to the end of this article.