not a question of mistakes. it's a question of having the right tests and whether or not people can evade being caught. remember, there is no test for HGH, even though it's a banned substance. so while a negative test result helps, it doesn't close the book. actually i don't know if the book can ever be closed anymore, for anyone.
to give an example why, marion jones was tested and passed every time. she was only caught because they found her name on a BALCO invoice...seven years later. one of the many things BALCO specialized in was teaching clients how to test negative even when they were positive.
it's all a bit of a conspiracy theory, of course, and has some of the worst aspects of a conspiracy theory in it--zero proof, circumstantial argument, denies validity of any proof to the contrary, etc. but of course i'm not claiming that it is the truth...only that i'm suspicious in this case and have a bad feeling that evidence will eventually emerge validating that bad feeling.