Transmission flush

If the transmission has not been serviced regularly a flush can cause issues by cleaning the deposits and varnish build up out. Normally this would be a good thing but in the case of a transmission it can be fatal. As the parts wear those deposits build up around and behind the various seals and rings in the transmission and actually help those worn parts to keep working. The flush of new fluid with nice new detergent can wash those deposits away and suddenly the transmission stops working or it starts leaking.
Have seen it happen more than once, someone drives into a shop, gets a transmission flush and suddenly the transmission either starts showing problems or stops working entirely.