I am not familiar with the IDPA safety officer procedures, however, the USPSA procedure is for the range officer to personally verify the chamber and magazine well is empty before giving the command "gun clear hammer down holster". This means a flashlight for the RO on a low light stage.
This was a fail on the part of the shooter and the safety officer.
I was that SO. Yes, the IDPA SO procedure is the same as the USPSA procedure. Yes, I had a flashlight. I still failed. I was tired and I saw what I expected to see, not what was there.
I suspect that many SO/RO have done the same thing many times. What has saved them is that they didn't fail at the same time that the shooter failed. But if the holes in the Swiss cheese line up (SO fails at the same time as the shooter fails), then racking the slide multiple times is likely wake them both up -- an extra round or two coming out of the ejection port is pretty hard to ignore.