Many years ago in a bizjet, started both engines off of the GPU. Gave the pull the plug sign and received a disconnected and good to go signal. (older model with no indication in the cockpit)
Moments after starting taxi had a frantic line man run beside and give the stop signal.
We shut down and discovered bent sheet metal and a GPU cord in 2 pieces as it had not been disconnected and we ripped the cord in 1/2.
This was at our home base and with a very experienced line staff. When I asked him what happened his response was " I can't believe I was looking at my partner and the plug was still in the aircraft by I gave you the all clear signal anyways"
Complacency, distraction, or brain fart, It could happen to all of us.