The one thing common in fanfics and internet fiction that I'm really tired of is when, despite the whole story so far being rather country-neutral, suddenly a character says or does something that only a person in the USA would say or do, and the country the story takes place doesn't matter or is supposed to be ambiguous.
Some examples (all of them I've seen before):
Refusing/not being able to drink alcohol because of being below 21, despite being over 18
Driving a car despite being below 18
Owning firearms despite not being a policeperson or a soldier
"How am I going to pay for treatment/an ambulance?!"
Easily changing their legal name or other personal details on demand (in most of the world this is either impossible or takes a very long time)
Saying that a relationship between someone over 18 and someone below 18 is illegal (in most of the world the age of consent is 14-16, not 18 or 21)
Easily getting non-life-saving medical procedures, such as plastic surgeries, abortions, or sex changes (in most of the world these are either illegal or require permissions of many doctors first)
Having to follow a curfew (although some other countries have curfews too)
Of course these things make sense if you write a story that specifically takes place in the USA, but in 99% of cases I see the country is never mentioned.
