Having previously been trained on the Gnat and Hunter, when I returned to Valley for a refresher course on the Hawk, I was taught ALWAYS to check visually that the seat firing handle was fully down when inserting the safety pin and that the seat and MDC pins shouldn't be re-inserted until the aircraft was stationary with the engine shut down on chocks - "It's a zero-zero bang seat and you might need it as a last resort on the ground".
But I read that people are re-inserting pins as part of the after landing checks - why? Particularly an inexperienced passenger fumbling about. I can't see how a pilot can visually check that the pin has been correctly inserted if he/she is looking where the aircraft is going whilst taxying.
Although the SOP I was taught might have prevented the seat firing handle being pulled inadvertently, the over-tightened scissor shackle would still have proved fatal for a zero-zero ejection with no time to use the manual separation procedure.