ive never had it happen quite like that before, but i have had some loads shift that were a major pita.
the most annoying is when im hauling corn and when i go to dump it into the unload auger, the corn will bridge in the bed of the truck and when i bump the bed up just a little, it will suddenly shift and flow over the back.
the worst i ever had was once as i was leaving the flour mill in mooresville with 4 pallets of flour centered in the truck. i took a turn too sharply and all four shifted to the left side against the wall. they cocked the truck over so bad that when i went back to the mill to get them repossitioned all too the front, we had to put a manual pallet jack in the back by hand because the truck wouldnt even fit up to the dock. (DISCLAIMER, before i get flamed too badly) i was only 17 and the loader put them in there that way and i didnt know that it was wrong.