There's been all sorts of odd accidents.
I knew a guy personally who always wore a 2 inch or so cross on a chain around his neck, had an accident and that cross went into his neck and buried itself to where the chain met the cross, it just missed his artery by 1/4 of an inch, so does that mean not to wear a chain with something on it? no, because I'm pretty sure that's never happened before or since!
I knew another guy back in the day of Walkman's, crashed and landed on his Walkman that heavily lacerated his kidney. Actually, wearing something hard on your side and getting seriously hurt from it in an accident is not as uncommon as it seems back when people wore those side bags, I haven't seen people wear those in a very long time.
This next situation is also more common than people realize, but a person crashes lands on his back, but in his jersey pocket was something hard, like a pump, or a multi tool, etc., and that thing damages a vertebra, I knew a guy that happened to as well and he could no longer ride a bike after the surgery.
There are weird accidents where anything can happen, but I would think that if a shoulder carry bag was causing a lot of accidents, couriers and others would no longer want to take the risk and find another way to carry a bag.
Speaking of weird accidents, not related to cycling, but to show how weird stuff can happen. There was a guy where I lived many years ago installed a set of wall mounted box speakers in his van up in the top corners of the rear of the van, had a crash, one of those speakers ripped away from its mount and hit him on the head and killed him. Or the guy in NY City who absent mindedly stepped out in front of a city bus moving at 30 mph, the guy went flying, landed, got up and was screaming that he was alive, and while jumping for joy stumbled into the street again and was then hit by a taxi and killed! Man, that could have been such a good day for that guy. I know there are millions of these types of stories, including my own, where I hit an embankment at over 140 mph, flipped over smashing the top about 4 inches below the dash; this was back in the day before all the crash stuff was in cars, though not sure if at those speeds that crash stuff would do any good, but the car was torn apart and looked like it had gone through a trash compactor, and I didn't even get a scratch, bruise, whiplash, or even a headache! There's a lot more to that story. So weird stuff can happen, good or bad.