An interesting case. Quite the bad luck to have the slide detonate the primer/bullet.
I use that 'trick' sometimes to unload, turning the gun upside-down*. More often (say 80% of the time) I twist the gun a bit more than 45 degrees so that sling-shooting it back causes the round to eject directly up and I then catch it.
*However, when I think about how I go about unloading the round (gun upside-down), I don't 'slingshot' the slide back but rather guide it back under control...and I also don't use this technique to clear any type of failure (jam, hangfire, stovepipe, etc, etc)