I have a lot of glass carboys. I mostly use them now for wine, since I hate hauling them around.
I've only broken one over the years, but my friend had a dangerous incident a few years ago with one of hers.
She was lifting a 6.5 gallon carboy up to the sink to rinse it out, and the top just bumped the faucet. It broke, falling down, and on the way it nearly sliced off her thumb. She grabbed a towel, and drove herself to the hospital (about 1 mile away) because she figured she didn't have time to call me.
She recovered just fine, with only a scar where they reattached it. The bill was thousands of dollars, mostly covered by insurance.
That really scared me! I think what happens is we bump the carboys many times incidentally, and then when they get bumped again just so they can break like that.
I use buckets and plastic fermenters for primary now for all wines, and all beers. I use glass carboys when I have to for wines but I'm very cautious and don't move them around except on racking day.