If i make a -.01 adjustment to my z offset while printing, the printer will push the print head all the way down into the bed. i shut the machine off and turn the z screw collar to raise the print head back up off of the bed. The other thing i have noticed is that when the machine is off if i turn the z screw lightly it will drop roughly one inch. Im not sure if the two are realated. sometimes i get to print, sometimes i get to tinker, such is ender 3 ownership i suppose.