Several years ago now, a man who worked at one of our power generating stations was killed in exactly the same fashion. Beetle-ing along with a smaller forklift meant for use on a hard flat surface, preferably indoors, and dropped a wheel into a pothole. It was enough to cause it to go over past the point of no return and since he also wasn`t wearing a seatbelt, he fell out and ended up with his head between the FOPS and the ground. Killed instantly.
You might think a forklift meant for outdoor useage would be immune to potholes, and they are...until Murphy happens along. Again, several years ago now, at the RR yards here in Sutherland, the operator of the seacan lift (looked similar to this one)
was in the yard carrying a 40 foot container and was about to stack it on top three or four others.. He dropped a wheel into a pothole that he had to drive through in order to position the seacan properly and laid the unit on it`s side.
The seacan was loaded asymmetrically so most of the weight was at one end, the end on the same side the pothole was, and the sideways rocking was just enough to put it over. Impossible for the operator to know about and he was being slow and careful but physics and Murphy did it to him.
Did a lot of damage to the unit too, bent the mast out of shape so much it couldn`t be used. They had to resort to trying to use two cranes to pick up the seacans and move them that way. You can imagine how well
that worked!

They had to bring in a spare unit from Regina...a 160 mile trip and severly overweight for the roads at that time of year (the RR needed to really suck up to Dept of Highways to get a permit to move it) and it was also a High Load Move for the power corporation, which is where I was involved with it.
The operators had been complaining about the potholes and telling management that they needed to be fixed, but as is the case with most management, they `knew better`and did nothing about it...until it was proven that it was indeed a hazard. A costly hazard at that.