My favorite one was pregnancy being a pre-existing condition so if your husband got fired (noone would ever fire a pregnant woman for no reason), you couldn't get insurance when you needed it the most.
Companies do this all the time. My sister worked at a major oil company that laid off all the pregnant women and new moms out on maternity leave within her department. (To be fair, they also laid off about 1/3 of the rest of the company as well.) It's totally illegal, but if you've just been laid off and have a new baby on the way are you really going to have the money or energy to fight a lawsuit? And even if 10% of people affected did, it's cheaper for the company to settle for $500K or so than to pay the salaries of all the people they fired.
Data seems to agree: in 2007 the usage of COBRA was 10%: http://www.cnn.com/2009/HEALTH/01/23/cobra.health.insurance/