I used to have my battery light and/or check engine light turn on when downshifting. The engine stalled itself out when dropping from high rpm to low (idle) and hitting the brakes at a certain time would cause the car's brain to think "I'm slowing to a stop at a light". This happened even at highway speeds. All I had to do was realize how I was causing it. I have rarely experienced it again by giving the engine a second to un-stall before applying brakes.

...because you'll see me passing everybody at 90 mph going up mountains.