Exchange 2010
So I've got a user who was looking back through her e-mail and noticed that quite a few messages in her inbox have attachments on them that are obviously attachments that were included in outgoing messages from other users. So User A in my organization has an e-mail from an external sender, the sender didn't attach anything to the message. But attached to the message is an attachment from User B in my organization, but this attachment was never sent to User A before so it's completely random.
The messages this is happening on are not new, but a few months old. User A says she didn't notice these attachments before.
This is only anecdotal, but it's particularly alarming because User A got a confidential HR attachment.