I've heard from many sources to check beans for stones before soaking/cooking them. I've been cooking with beans for years (mostly black beans, chickpeas and lentils), and I've never encountered a stone before or after cooking. Is the stone thing a myth, or no longer the case with modern agricultural technology?
I've found stones in dried beans, so it's no myth. Not common, but I'd say I find one every year or two. If you simply swallowed a small stone, it would almost certainly pass without harm, but as TFD pointed out in his comment, biting down on one could be an expensive and painful dental experience. What I do is spread the beans out on a kitchen towel in a single layer. For dark beans, I use a light-colored towel and vice-versa for light beans. It's very easy to spot any foreign objects that way and it takes less than a minute, so it's worth doing in my opinion. You can then easily toss the beans into a pot by just picking up the four corners of the towel and dumping them in.
I should add that I've found little balls of dried dirt far more often than actual stones. Those would most likely dissolve and get washed away when you rinsed the beans, but it demonstrates that stones can be in there.
In Mexico I get stones all the time. Thankfully they are normally not as hard my teeth so I havn't broken a tooth yet.
I havn't had this occur in Canada only when buying them in Mexico. Almost every pack has atleast one if not more.
Flor de Maya was the most recent one I got stones in. I think it is more common in Mexico.
Quite to the contrary - it is the modern agricultural technology that causes the stones. Modern methods would mean of the past 50-60 years and longer-combines have been used for many many years now. Hand harvesting would of course be picking beans right of the vine would not yield any stones.
When I make pintos I find no less than 5-6 stones. When I make field peas I don't find anything. What's the difference? Pintos are store bought. Field peas are home grown, hand processed.