I had a strange incident a few months back in the -800. We were in the descent somewhere in the FL200's with the speed brake fully extended at flight detent. As I slowly retracted the speed brake while we were in a shallow enough turn the autopilot disconnected, then the aircraft banked to 40 degrees and we got the 'Bank Angle' warning. I would guess that with the speed brake not fully retracted it caused the high roll rate, just as the book says however it was still a bit frightening as the aircraft really snaps into the roll. Another theory is that it was a strange aircraft as we had a generator failure on the next flight
