"or doing lat pull downs, thinking you have the full rack 350+lbs and its only on 40lbs and you fall on your ass comign down with the bar."
I did the exact same thing with the stack and seated cable rows. Let me guess, the plates at the bottom were thicker but you accidently used the short pin, so it didn't engage?
I flew backwards a good 8 feet and landed on the back of my head.