Didn't happen to me, but a friend that worked for the same company as I did--
We worked for a lumber broker, hauling dressed southern yellow pine pine lumber. If you don't have SYP in your part of the country, it's a softwood used in just about all of the construction here. When dressed, it's got a very smooth, slick surface. Put into bundles, and with a little sawdust on every board, it's extremely slick and sometimes hard to keep the center of the bundle from walking out when hauling it. I have had the entire center of a bundle walk backward and fall onto the trailer, and have had it come forward in a quick stop, also. Now as to what happened to my buddy--
He was going up a two lane road about 2 AM, a dark night, no moon, drizzling rain. A very black cow decided to take a night time stroll out of the fence to get to that grass that is always greener on the other side. You can imagine what happened next, they were both on the same piece of real estate at the same time, he braked hard as soon as he seen her, but it was too late, he couldn't get stopped in time, BOOM he hit the cow. Knocked the cow down the road, she rolled over, ended back up on her feet, and his momentum carried him right into the cow again! BOOM again! I should add, he was moo-ving on when he hit the brakes
. Anyway, sometime, either the first hit or the second, he never really knew which, the center of the pack of lumber on the top of the load didn't stop when he hit the cow, it kept on coming.
The top layer was above cab level, and most of the lumber went over the top of the cab, over the hood, and onto the ground. We had R model Mack trucks, day cab with the back window, no sleepers, and no headache rack on them, only a headboard on the trailers that only went as as high as the second layer of lumber, the third layer didn't have anything to slow it down. Most went over the cab, but two 2"x10" boards came through the back window, one stopped when it hit the dashboard, the other continued through the passenger side windsheild and knocked off the turn signal on the fender. If it had of come through on his side, he wouldn't be with us today.
The boss didn't get us sleeper cabs until several more years after that. I was always nervous when I had to make a sudden stop after that, waiting on a board to come in with me, but luckily it never did!