I work in flood restoration and I see it pretty often, but it's usually on older pipes from the 1960s or older. The most interesting ones are when the water or minerals in the water have worn through the corner of the pipe.

The ones that I've seen pop off or separate are due to poor installation.

But also you have to think about how often the above occurs compared with how many joints exist. Something can be both incredibly rare and happen consistently. As in a failure rate of .05% is great , but .05% of 1,000,000 is still 500.