I found an email in my sent mailbox which had gone to a completely different and unrelated address from the one I'd entered by tapping the reply button on the original email I was answering I hadn't gone anywhere near my address book so the change of the intended address is absolutely inexplicable. Is there a bug that does that? I'm using the Apple Mail application on a new Macbook Pro with Yosemite.
This is probably connected with a "reply to"-address provided with the email you received. For example, many companies use this when sending automatic generated e-mail from an address that doesn't receive e-mails. When you reply to such mail, your application checks if there is a "reply to"-address provided in the message's head, and if it is, it will choose that for your reply over the senders address. If your mail was not from a company but from a friend, this could be a bug or mistake in his mail application, maybe he provided such a reply-to-address to. Some applications do this when a feature to collect all mail in a single inbox is chosen. If none of these apply, you should check the authenticity of the sender's address, and maybe talk to him on the mail you received, as this could also be a sign of spam messages.