Understanding the instructions.
My father used to teach in the Royal Navy. He would get a classroom full of people from the Commonwealth Navies and teach them. At the end of the first class, he asked if everyone understood. Most people nodded, and a few shook their heads. The people who nodded were allowed to go.
My father did the whole lesson again - much slower. At the end, he said "do you understand" and the class shook their heads.
Ever patient, my father prepared to do the course again, when one person said "We understand perfectly, when you ask use we all said yes".
In their country nodding means no - and shaking the head means yes!
As well as this little confusion they did not like to say no. The answer to a question "are you on target and ready to fire" was always yes. They did a lot of damage by hitting things outside of the target area.
Another problem was caused someone not using his right eye to line up the back sight and the fore sight, but using his left eye and only the fore sight, and shutting his right eye! This makes a big difference over a couple of miles.