There's a big difference between just putting weight on the rack while standing still, and subjecting it to the vibration, potholes, and other dynamic forces it will experience out on the road with that same load on it. If you actually ride the bike with anything like 150lb on the rack that's rated for only 40 or 50lb don't expect it to last very long at all. The signs of rack failure will be cracking at stress points if you're lucky, if grossly overloaded it's more likely to collapse with little warning as soon as it breaks at one point and the additional load on the remaining points overwhelm them.