I've identified two universal paradoxes. Both are fairly commonplace, but to my knowledge, no one has formally identified them yet.
The Weight-Training Equipment Paradox: If you can carry it into your house, you don't need it.
Years ago I purchased a barbell set, and was embarassed to discover that I couldn't carry it from the display shelf to the checkout area. They had to bring a cart for me, and two guys lifted it into my trunk. When I got home, I opened the box in my trunk and struggled to carry the weights into the house one by one. If a person is strong enough to lift that box, then the only reason they would carry it into their home is to gift-wrap it.
The Professional Forecaster Paradox: If you're any good at it, you don't need to do it for a living.
There are two professions I can thing of where someone works at making forecasts that they then sell as a product to other people. The first are stock analysts. If they're so good at picking stocks, then why aren't they rich enough to retire and buy an island in the Caribbean? The other are racetrack handicappers. If they're so good at picking winners, why do they need to sell their picks?
Can anyone suggest any other commonplace paradoxes?