It's got to be pretty unusual to have a drug not cause any problems during animal testing and affect humans so profoundly. In the coverage that I heard this morning, they said the docs had no idea why the people were having such problems, and they could do nothing but treat the symptoms.
Hopefully this won't make drug trials any more difficult than they are already. There are so many drugs that could save lives that aren't available. I fear that such a high profile disaster will keep even more effective safe drugs off the market as the drug companies scramble to conduct ever more rigorous trials.
Good article on the topic in The Economist a couple of weeks ago.