I've had my z on jack stands many times and this has never happened. I'm assuming you didn't slowly lower your z onto your jack stands. Because you need a lot of force to bend the scissor tabs. You can also buy rubber pads to fit on your jack stands. But I guess it's too late for that since you bent them. I think its safer to place the jack stands where they belong, closer to the outside of the z where the weight is more balanced than finding your own spots.
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