If you use inbox folders to organize your messages, you may have noticed that when you click on one of those messages, the "historical" view of the conversation was showing all messages as being either from you or that person (but not both).
Mario's Right Nut uncovered a nasty bug that actually corrupted some data. Any messages that you've got in folders will never accurately report "who" the messages was from. This has been corrected, but the fallout is that those messages are permanently tainted (~1,500 messages out of 400,000, so it's not that big of an issue). You'll also notice that all of you PM folders are now empty and you'll need to repopulate them.
Sorry if this bad news ruined your day!