I've see a number of floors give way while my forklift drivers are in the trailer! Fortunately none went completely through the floor but around here there's a lot of salty water that gets thrown up onto the underside of the trailers and weakens the structure and the flooring. I always tell them to ask for the inspection certificate and if it's more than a year old we refuse the trailer (and load if there is one) and tell them to send it in an inspected trailer. You can see what Frank S pointed out, there's not much structure to these trailers and you can see the floor from underneath. This was one of the trailers that failed on us:
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