I hate when someone will pass along (or include others in a reply) when you may have typed something you meant to be personal between the two of you. Have you had that happen? Then as the email bounces back and forth amongst all parties, it gets ongoing air time each time. I should know better - yes.
But once it happens, they may not think it's a big deal or perhaps are making me pay for it by displaying it for all to see, including the person I may have made the comment on. Ouch.

When it comes to subject lines (or lack thereof)... I get those from only a few people but it's consistently them. And I don't know about you but I store my emails if they are of any importance. Months later I have to open each and every one to find the right one.