In two consecutive races, my girlfriend and I have broken our rear derailleurs in similar fashion. We're both riding 2011 Kona Jake the Snakes (aww, how cute, matching bikes...). Her's was built up from a frameset and has a Shimano 105, mine was running the stock Ultegra. Mine broke at the Cross Crusade race at PIR, where I met another 2011 Jake the Snake rider with an identical broken derailleur; and her's broke at Kruger's Crossing.
Limit adjustment and chain length had been checked, and both derailleurs were shifting well. Whatever the cause of the damage, we'd both have preferred to have the derailleur hanger sacrifice itself to save the rear derailleur. Is that an unreasonable expectation? Are the Kona derailleur hangers too strong?
Samething happened to me at Krugers. I'm running SRAM Rival. The hanger's fine.
What method are you using to check the limit screw? Chain length doesn't factor into this.
I think it's normal, unfortunately. A lot of mud build up will tear a derailler right off. That is odd the hanger didn't shear off and the derailler did. Might be a bit too beefy, I'm not sure.
This is where my boss's dictum of "two crappy bikes are faster than one nice bike" comes in. If you're at your financial limit that's totally cool and understandable but having two low end bikes is nice. Even if you just do one bike exchange you can usually avoid this sort of thing.