openssh
Give info about ssh-keygen -R to remove offending key
Giving a sed command in the error message about offending server (host) key allowing to just copy and paste the command in order to remove the offending key.
Firstly, sed is the wrong hammer for this particular nail; ssh-keygen has -R which does this. Secondly, we have been historically reluctant to provide a way for the user to blindly do this since it enables MITMing of subsequent connections.
There's a bug requesting this: https://bugzilla.mindrot.org/show_bug.cgi?id=2045 however due to the aforementioned reasons it's not seen much love.
I changed the info to display the use of ssh-keygen -R instead of my previous sed proposition. I also replaced the display of the host so that the command cannot be completely copied and pasted anymore.
From my experience not a lot of people know about the ssh-keygen -R command, and are using sed, or worse, are deleting completely the known_hosts file (as the complete path is given in the error message and can be easily copied and paste). Even if we're not giving a complete command to copy and paste blindly, adding the info about ssh-keygen seems appropriate to me.