One wrong click. That's all it takes to breach privacy and reveal the personal information of every recipient on an email list.
Just landed a great internship at the White House? CC: your mom on it in a reply and your bosses just might wonder about your ability to keep things professional in the workplace.
According to a 2011 poll by digital communication company VaporStream, 60% of employees occasionally hit “reply all” on e-mails when they intend to respond only to one person.
Think carefully about who you CC: or BCC: on your emails. Remember that when you CC: someone, you're revealing their address to everyone who receives the message. This could lead to a slew of problems from undesirables harvesting emails, to office embarrassment upon revealing personal tidbits to undesired recipients.
For more on how to avoid potential technological blunders, see this article from Inc. entitled 25 Tips for Perfecting Your Email Etiquette.