I nearly accidentally “reply all” a few times a year when I only want to reply to the sender. While it’s not earth shattering, it quite could be! For the most part, it’s only slightly embarrassing.
Outlook, Gmail, Hotmail, Yahoo mail, etc… should have a catch for this. A “Are you sure that you want to send this to 100 people?” warning would be sufficient.
Gmail has a late night drunk test that you can turn on. One would think this would be better and more used.