Clarke's Laws are famous; the first one is
When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
Today, after a discussion about what could have caused a network outage in one of our various offices (I suggested a localized power problem in the building but my boss thought that couldn't possibly be it — and he was wrong), I propose a new addendum based on Clarke's First Law:
When someone suggests a way something could go wrong with a complex system, they're almost certainly right; when someone responds that that could never happen, they're almost certainly wrong.