Sometimes host keys change and we have to remove their corresponding entries from .ssh/known_hosts. Instead of doing that manually, you can use ssh-keygen with the -R option:

$ ssh-keygen -R foo.com
# Host foo.com found: line 1
# Host foo.com found: line 4
/Users/gh/.ssh/known_hosts updated.
Original contents retained as /Users/gh/.ssh/known_hosts.old