I remember driving behind another car on a two lane highway, and could tell he had something tied under the car. This was in the 1980's so cars were much longer and had more framework. Under the car sticking out both the front and back were a number of lengths of pipe. Some how he had strapped these pipes to the underside. This is one of those things that will work till it doesn't. I quickly scooted around him because I did not want to imagine what would happen if he hit a pothole and a length of that pipe caught on the highway going 60 miles a hour.
I have broken some rules before. I had a ranger and with support boards and counterweights (my good old 120 lb anvil) I hauled 20' cedar siding. Even with a sixteen ft 2x10 for support, it still flexed enough to sometime hit the street. Remember, with this much leverage, it will pull the rear of the truck down. I could not back up because the siding was that close to the ground.
At night I was carrying a metal light pole post (from a gas station). I had it all strapped in with a light on the back (always have a red flag tied to anything sticking out!). When I tried to back up, it felt like the transmission was being yanked out of the truck, and the pole jerked. I tried this a few times then got out to see what was going on. The surplus length on one strap had come un-bundled and was flapping behind the truck. When I backed up I ran over this loose end, and it tighted and yanked on the pole, and the pole would jam into the back of the cab.
ALWAYS USE A RED FLAG! I had re-bar properly marked and was sitting in a drive-thru line. The car behind me kept getting closer. I moved forward as much as possible, but the person behind me ran into my rebar, jamming a piece between headlight and grille. I had the flag and she admitted it was her mistake, but I still felt bad.