Around the same time that I got this new laptop, TAMU switched from hosting its own mail to a Google Apps-based system. As part of the changeover, you can import your mailboxes from the old system to the new system. After hooking up the new account in Mail.app as an IMAP account, there are messages where what you select is not what you get: The sender and subject are for a different message somewhere else in the mailbox. In fact, this is the case for all of the imported messages; new arrivals seem to match up. There are also many unrelated messages grouped as if they belong in the same thread.
Now, blaming this on Mavericks might seem unfair, as there are multiple partners involved here: A&M’s import system, Google, and Apple. But this mismatch doesn’t happen on the old laptop running Lion, or on my iPad or iPhone.
Various posts online suggest rebuilding the mailbox. This didn’t work for me. In the web gmail, I deleted the label for the import, unstarred everything and marked everything a not important. I did another rebuild… not joy. Rebuilt the Spotlight index. I think that may have fixed it.