One possibility I haven't seen mentioned yet: if you previously had your cruise control set to something high (say, 70 mph) and then unintentionally activated it when driving at a much slower speed, the sudden lurch forward as it tried to accelerate to the set point would certainly be disconcerting. And of course, hitting the brake would cancel it, just as described in the first post.
I had this happen when I was first getting used to the Model S cruise control, so I wonder if this is also happening to other people.
Well, the 3 doesn’t really have “sticky” cruise set speeds (much to my disappointment). So it would have to think it was on a road and had a speed limit in the display for the cruise to have accidentally accelerate like what you are talking about. Possible, but doubtful the 3 was recognizing a speed limit in a parking lot.
The mechanism to make that happen is there though, if the OP accidentally pulled the drive stalk again. Like if it had a delay showing the car in D after the first pull, so you pull again thinking you didn’t engage it, but that actally turns on TACC.
However, I believe the OP said they removed their foot off the pedal and that stopped the acceleration. Cruise wouldn’t do that.