I'm 100% sure that no one has ever fired a gun while cleaning it, its just an excuse people like to use
I know of a few. And they all should have known better.
People get complacent when they do ANYTHING a lot, and that includes handling firearms. I am a certified RSO and work RSO duties at the ranges for the club I belong to. I have seen several NDs at the range. But of course, they were not cleaning their guns at the time.
Every one of the gun cleaning NDs I know of happened when the guy 'thought' the gun was unloaded. They removed the mag but did not eject the round in the chamber. Some guns, like a Glock, require the trigger to be pulled before removing the slide. That is when each of these NDs happened.