Better leave the back pack where it cannot fall. Hindsight is so much better than foresight . I learned the hard way too!

Don't know about ruined screw head removal, but for future reference: Place the tool lightly on screw head & rotate axially 'til the tool drops into the screw head for a comfortable, firm fit, before applying pressure & torque for screw removal. If the tool does not drop in the screw head for the comfortable, snug fit, you may have the wrong tool or the wrong size. Try a different size 'til U get the right fit before applying pressure & torque to remove the screw. It can save you grief from a ruined screw head.