What was intended only for one special recipient got an audience of many. Unfortunately, this can only too easily happen. And the opposite, while not as embarrassing, is possible as well.
This is why you should
- always, always check the To: line when you compose a reply to a message that appeared on a mailing list.
Many lists are set up to automatically use the list address for replies, some are not. Some email clients treat some mailing lists in a special way. So... it is generally a good idea to have a look at the To: line and see if the message goes to the intended audience.