In a previous life I had to inform volunteers who used the IT systems of the charity I worked for that if they had to send an email to over 5 people (which is very common within this charity) they shouldn't put all the recipients emails in to the To: field or the CC: field, and if they could they should put them in the Bcc: field instead.
I had this one thick (as in deliberately obtuse) person flat out say she wouldn't do what I told her to do. In front of 20 other people. I asked her why, she said she didn't want to. I ask her why again (two can be obtuse as well dickhead) and she repeated her answer. I ask her 2 more times and she fires up "Oh my God I told you already why do you keep asking the same question!?". I answered "Imagine now that was an email you sent to 200 people, all of their emails in the CC field, and every one of those recipients hit Reply All asking you why you didn't use the Bcc field and why everyone now has their email address, and why can they see everyone's reply".
She looked fairly stupid, especially with the 20 other people sat around looking either a bit awkward at her dumbness or smiling that she had hung herself with her own dumbness.
Biting the hand that feeds IT
