When you send an email to a list with a file attached, but you attach the wrong file. So a helpful soul replies with “you attached the wrong file” and you want to just ‘reply all’ and attach the correct file – but the helpful person didn’t want to cause you shame by publishing your error, and so their reply was private.
Now to send the correct file, you have to dig up your old email from the sent items folder and reply to that with “sorry for being stupid”.
So – it helpful friends: when you want to help, please use ‘reply all’. It’s the friendly thing to do, as it saves me both annoyance and face.