I've only broken one carboy, and it was because I dropped it. No injuries, just a mess. But my friend had a visit to the ER when she broke one. It seems like the breakage is random, but it really isn't. It might get bumped several times and not break or show any damage- but then you just lightly bump it against the sink and SMASH. That's what happened to my friend- she had hauled a full 5 gallon carboy to the basement by herself, let it ferment and then racked it. Then picking it up to clean it in the basement sink, she lightly rapped it against the faucet. She drove herself to the ER (only 1 mile away) to the hospital where I work. Lots of bruising and quite a few stitches on her wrist. She had a big bandage for a while. She said there was lots of bleeding.
I think carboys are pretty durable, and I bet I must bump them or use water that's too hot or something, and they show no signs of breakage. Then suddenly, they might break. The straw that breaks the camel's back. I have one 3 gallon BB that I've never used (I bought it for wine), and one 5 gallon that I use all the time. The BBs are lighter to carry and unbreakable. I'm going to have to get some more.
Lorena