A slim chance and a fat chance are synonymous, but a wise man and wise guy are not. Overlook and oversee are opposites, but quite a lot and quite a few are similar. Some concepts are important only in the negative. Nobody would ride in a horseful carriage or wear a strapful gown. We have no sung heros.
Nobody writes songs about requited love.
You've probably never met anyone who was combobulated, gruntled, ruly, or peccable. Your house can burn up and burn down simultaneously. Your insurance company will want you to fill in or fill out a form. It doesn't matter which. They're the same.