How about...
Before "Mad Cow" disease, everyone thought the odds of an infectious protein were zero.
I personally don't beleive there is surface (or even near-surface) life on Mars. I don't beleive that any life on Mars is capable of infecting any life on Earth. but I can't rule out a tiny possibility, and that COULD lead to a massive problem.
So, although I chafe about the difficulties and expense that a quarantined MSR causes, I accept its neccesity.
Think about thew consequences, tough. Because we have to send and retuirn a more complex and heavier capsule we need a bigger launch rocket. When we have proven the risk to be infinitesimal, we can dispense with the multiple containments and use that increased capacity as useful load for future missions.