Wearing Shoes Inside
This is a pretty gross habit, actually. Americans have normalized walking around the house in their shoes, and they don't consider all of the outside germs they're tracking around the carpet. In countries like Japan, you have to immediately take your shoes off if you even want to step foot in somebody's home.
Calling it "Tuna Fish" Instead of "Tuna"
One Reddit user made a good point with this one. "Why do Americans say 'tuna fish'? Is there another species of tuna that I'm not aware of?" Where did the nomenclature for this come from? Why do we overly confuse everything?