Ever drive a 70 mile round trip to pick up pipe and fittings?
You thought you were careful when selecting from the bins, but when you got home, there was this one Odd Duck, which is a key player in what you wanted to do that day?

Go back tomorrow.
Not the same thing but I now always bring the fitting, hose section, bolt, nut etc. with me to the local Ace to make sure I get the correct corresponding part, or I invariably have to make a second trip! Eyeballing doesn't always work for me!
And people do put the wrong size back in bins especially at the hardware store!