I've never cracked a weld on one. What usually happens when manual pallet jack is overloaded is the forks get bent outward....still works but forks are forever not parallel to each other anymore.
I've overloaded many electric pallet jacks, with no harm to the jack but they are typically built way heavier duty than manual jacks. Many times have used electric pallet jacks as motorized mules to move long heavy machines that are on skates at the other end. Sometimes had to lower the pallet jack end on the jack forks with a toe jack as the weight was beyond the capacity of the jack to raise the load, so the jack was for motion purposes only.
Overloading a manual pallet jack seems silly due to muscle power needed to move it around. I always preach the charms of electric pallet jacks but few seem to listen. Manuals do have the advantage for certain machines however...like where the battery of an electric would get in the way so you can't get under the machine as far as you'd like. There is a solution for that as well however...use short height deep cycle golf cart batteries instead of the tall typical pallet jack battery.
I have a 10,000 lb manual pallet jack by Lift Rite but I ever use it anymore...should sell it... try moving 10,000 lbs around by hand....can be done on a perfect floor, but you end up doing more pry bar moving the jack than normal pulling.