Holy Mother!

In what situations, do you hear the Chinese say "oh my mother 我的妈呀"?

从前有一匹马，只要对它说:我的妈呀！它会以一秒千里的速度飞奔，但只要你对它说一声:我的天呀！它会骤然停下。一个商人看上了这匹马，便买了下来。他说了一句:我的妈呀！那匹马飞一般跑了起来，果然飞奔如野。没多久，他们便驶入了距城中心30多公里的丛山中。商人忽见前方有一悬崖绝壁，立想停住这千里马，但忘记了使马停住的口令，我的地呀！我的祖宗呀！，就是忘了我的天呀！在千钧一发之际，他终于说出了:我的天呀！马在离掉落只剩几厘米处停下了。商人叹了口气，感慨地说了声:我的妈呀！...

Once upon a time, there was a horse, and if you told him, "Holy Mother!" he would run at the speed of light, and if you told him, "Holy Cow!" he would halt just as fast as he ran. A merchant saw this horse one day and took a liking to it, and bought him (for only ten million dollars to boot). "Holy Mother!" the merchant exclaimed, and the horse immediatley started to break into a gallop, and he really was worthy of a wild rabid beast. A few moments later, they were a well good distance away from the city and at least 30 kilometers into the mountains. Suddenly, the ledge of a cliff came into the merchant's peripheral view, and he knew he needed to stop the horse, but, had forgotten what the command was (oh crud), "Holy Nature! Holy Ancestors!" He said about everything that began with 'holy' (yes, what a potty mouth!), but Holy Cow. And just when his life was flashing right before his eyes (a pretty wife, four children, and living to a ripe old age), he finally shouted, "Holy Cow!" The horse abruptly came to a halt just inches away from the edge of the cliff. The merchant let out his breath, not realizing he was holding it and his face was turning blue,"Holy Mother!" he exclaimed in relief.