DANGER - Facebook Messenger
Imagine your email software has this feature: As you are reading an email you are corresponding with person A, there is a Add button. By clicking it and selecting another person B, all the emails you have with person A in the past are immediately forwarded to person B, without warning. The email forwarding is executed virtually. Instead of physically forwarding each mail one by one to person B's mail server, basically your mail software gives access to Person B, at his own leisure, to browse EVERY of your email with person A. If you had done this by mistake, there is no process to cancel because the "forwarding" process is complete the moment person B is selected. There is no Undo option. You can scream and run round the room, but person B will continue, at any time in the future, to be able to read EVERY single email you had with person A. If you are a careful person that would never do such a silly thing, person A could click the Add button and the outcome will be...