This happened to me a number of times: sometimes, when I reply to an email with multiple recipients, instead of replying to all, I reply to the sender. Is there any way to prevent to do it? e.g., a Mail.app plugin that warns you about such an issue?
A quick-fix way to avoid this error is to Control-click on the Mail toolbar, select Customize Toolbar ... and instead of accepting the Apple default where Reply, Reply All and Forward are joined in one big button, choose each one singly, dragging them to the toolbar as a single button, spacing them widely and not contiguous at all ... and Show Icon and Text makes it even more obvious which is which.
If Reply All is the one you use most often, set the spacers so it shows up in the middle, and place Reply to one side. I do the same with Junk and Delete as well.