A lot of these types of 'cleaning' incidents happen due to people removing the magazine on an semi-auto Glock or similar type weapon and then think the chamber is empty. I have seen this before because they normally leave the chamber empty and think that once they drop the mag it is ready, but in reality have at sometime chambered a round. They go to remove the slide, and since you are required to pull the trigger before the slide will release, the pull it, failing to point it in a safe direction, and the gun fires.
Just remember that even if you are one of those that leave your gun with a round missing from the chamber, that it is STILL loaded and must always rack the slide to make sure you or someone else hasn't racked the slide and chambered a round without realizing it.