Why have I received new emails that have old email content (from separate sender) in them?

This is a weird bug, I think.

Twice today I've received emails from people who really intended to send me messages -- and their headers were correct. But when I opened the emails, the content was that of earlier emails (one that was sent to me this summer, and another I sent to a group in September).

It's like a gremlin got in there and switched out the content.

Any idea why this happened or what I can do about it? I have a one-year old MacBook Pro Mac OSX Version 10.5.5 with Leopard installed on it.....

Thanks.