Everytime this video circulates I watch it two or three times get my chuckles the realize how REALLY simple it is to have this happen.
In a way he did everything right except remove the magazine. He locks the gun back, looks at the chamber to assure it is empty then has a second person VERIFY that the chamber is empty.
Of course that is when Mr. Cool has to hold the gun over his head and click off the release to let the slide snap forward loading the gun from the still inserted magazine. Had he racked the slide in normal fashion, perhaps he would have noticed the usual feel or sound of a shell chambering.
But then the stupidity occurs. It IS a Glock .40 so it won't fire unless the trigger is pulled, so what is he doing pulling the trigger in the position when it fires. What was he doing at all in the position it fired ???
But still it brings up the point that just because you can see the chamber is empty does not mean the gun is not loaded.