Via Boingboing, a link to an article on two types of knowledge, both important in the creation of new products.
The article features an air pistol with an air-pressure gauge built into the reservoir. The problem is, the gauge is at the end of the gun, directly below the barrel. You must point the gun directly at your own forehead to check the pressure.
Technically, I don't think you could put the gauge on the side. I am not sure where you could put it, but if a pistolry expert had spoken with the design expert, this design might have been cancelled.