I've had a couple of clerks go sideways on me for trying to return money that they erroneously gave me.
The worst time was a guy at a 7-11. He screamed and yelled that once the register was closed, it was too late to correct an error. I merely politely mentioned that I think he gave me too much change. Afterward I thought about it and realized that this guy deals with winos, bums and middle school kids all day. He probably gets 10 people trying to scam him per day. It was simply beyond the realm of his comprehension that someone could be trying to help him out.
In my experience, pawnshops are similar. They deal with people who are trying to scam them all day. Of course, 95% of people in there are fine folks, it just the 5% that are meth heads and trying to pawn their stolen stuff, or trying to wheedle a nickle out of the pawnshop. The pawnshop employees get used to dealing with these people, and it colors their interaction with all the customers.
-John