I lost the plastic under-tray on my Peugeot 306 when I grounded on a very slight hump-backed bridge (and not even a sharp one) at maybe 50 mph. I had to drive with the tray scraping on the road to the nearest garage where they lent me a Stanley knife to cut it off its remaining mounting bolts, and then got my garage to tidy things up next time it was on a hoist being serviced. I drove it without an undertray for the next 8 years, having decided that a new one was too expensive. I think the tray may have already become detached at the front, and that is what let it drop far enough to catch on the road - and the hump combined with the speed meant the wheels were pushed up more than normal for a brief instant.
The car is not low-slung by any means, but I've grounded the front and back bumpers when crawling forward onto a steep drive, as it transitions from the level road. I also did the same thing with our Honda CRV (which is high slung, so should be even less of a problem) when we went to view a house that we were considering buying. We got there, tried to reverse onto the drive, scraped the front bumpers or skirt in front of the wheels on the ground as I was edging back very slowly (I could see that there might be a problem), and decided "this would be a liability, and my Peugeot would be even more of a problem" and aborted the viewing. The house owner and estate agent were not happy but agreed that it was for a very valid reason. Goodness knows how the house owner got his car on and off his drive without taking out the front or back bumpers/skirts is a mystery.