Once had an Assym. and that was on extension on the ground while taxiing for take off. Lockheed L188 Electra. Caused by failure of screwjack nut.
Just remebered another one on Electra caused by a very firm landing, I think one of the detection micro-switches bounced.
Had several Slat and Flap Jams but no Assym. on A300 caused by water ingress in screwjacks and freezing in flight, hence won't extend. Usually cleared by flying around at low level for a while, but not always.
Hope this helps.