The quad had just come off the bench after some tuneups and I guess I didn't tighten one of the props down enough. Had a couple surprises in flight... first time it did a quick dip, and I wasn't sure what had just happened. Instead of being sensible and landing right away, I did another pass, and this time the quad almost ate it. Turned out the prop was just a bit loose, so I guess it would slip and then grab again. Looking at the logs I could see it happening a few other times very quickly, but I didn't notice it in flight.
What was impressive about all this was how well the AQ recovered. At one point it was rolled ~85° and pitched ~35°. I wouldn't try this very often, but I was pleasantly surprised.
Here is a Google Earth render of the event from several angles and at different speeds. It's the right front prop that slips. Best viewed full screen.
Safe flights
-Max