my cars done that from time to time. some theories on having grip and slowly building rpm's gives the ecu a chance to catch the revs before they get out of control.
but if you go and bang out some donuts, or start sliding around in the rain, it starts bouncing...
i think you're being too gentle after your 'oops'
fortunately, it was the cheapest money shift ever! i did the same 5th-2nd shift at about 50. i was reaching for 4th and revved for 4th, so it was still a shock when the shift light banged on and the car lurched...