If it says "one size fits all," it doesn't fit anyone.
If it weren't for the last minute, nothing would ever get done.
If it works, don't fix it!
If idiots could fly, this world would be an airport.
If more than one person is responsible for a miscalculation, no one will be at fault.
If Murphy's Law can go wrong, it will.
If not controlled, work will flow to the competent man until he submerges.
If on an actuarial basis there is a 50-50 chance that something will go wrong, it will actually go wrong nine times out of ten.
If only one price can be obtained for a quotation, the price will be unreasonable.