So what is this mistake? It is sending group emails with the recipient addresses in the 'to' line or 'cc' line. Not everyone wants others to see their email address especially if they don't know others in the group message. This is where the 'bcc' is useful. Bcc stands for blind carbon copy. This means that no one can see the other email addresses of the people in the group that the email was sent. When sending a group email type your own email in the 'to' line.
Of course, there is a purpose for using 'cc', the carbon copy. This is where you are keeping others informed of correspondence and the members all know each other. For example, where there are three or four students working on a group task together.
So, it is important to think about who you are sending emails to and whether the 'cc' or 'bcc' line needs to be used. It might save some emails from upset recipients later!