The screws on one side thing is real but that's not why it went from level to rocking. Wood shrinking a tiny bit won't make a bench rock like that. What happened is the legs were not sitting flat on the floor to begin with and once you started pushing on the vise the whole frame racked and settled into a new shape. A bench needs to be stiff, not just screwed together, so you want braces going both ways on the legs or it will keep moving on you. Also check if your floor is flat, because a 1/8 inch slope in a garage slab will do the same thing to a bench with no diagonal bracing. Wet lumber makes it worse but even dry 2x4s will rack if the frame has nothing stopping them.

Man I did the exact same thing for way too long and just thought my TV was bad. You probably saved a bunch of people from buying a new controller for no reason.

That jam story got me, what a sweet memory to have.