Microsoft breaks DKIM signature?

It's kind of rare, but not rare enough. Every now and then I hear of a client who is seeing intermittent DKIM failures at Microsoft Outlook.com/Hotmail properties. A Delivery Team Lead for one of the bigger ESPs posted about this on the Mailop list recently, looking for feedback and thoughts. The discussion that ensued seemed to come to a consensus that there are (rare) times when Microsoft may be modifying message content slightly, and thus causing the DKIM signature to break.