I have never seen a hitch break, but I have seen:
1. Cracked frame on the tongue of my 2005 TM 2720 (well documented around here in 2006). I am pretty sure the failure was a combination of a bad weld for the bracket for the swing away tongue and the load at that point, especially during bumps.
2. I once watch my neighbor haul some old broken up concrete to the dump. The bed was loaded to the top of the rear window. I have no idea what the weight rating is for a Hudson truck (remember those?), but the rear springs were fully compressed and the rear bumper was nearly scraping the ground. His route to the dump included Devil's Slide in California. Some people do the darnedest things. How many air bags would have been necessary to level this load?
3. When I was a Boy Scout Explorer we had a home made trailer that we used to haul gear, towed behind a 1965 Jeep CJ5. We did not have a hitch failure, but the tongue bent enough that it scraped the ground. We used a hi-lift jack to straighten it out. We jacked up the low point and then put 300 pounds of scouts on each side jumping up and down. We strapped the jack (3 feet long) to the tongue using bailing wire. We might have been overloaded. Or it might have been from towing at 45mph on dirt roads with gullies.
All of these are unusual situations. The first was an engineering design flaw. The second was clearly a violation of weight limits. I am not entirely sure what the third failure was caused by.