I have had friends who have been victims of "switched bills" /scams using all three options.
What my friends experienced was hundreds were given to be exchanged & one (or more $100 bill) was declared counterfeit.
The bill was indeed counterfeit, but the provider switched it by slight of hand, under-the-counter, etc.
Having unknowingly accepted a "fake bill"; what are the vendors choices 1) accept the loss which could be a several day's income or 2) pass along to an expat/foreigner?
To protect yourself, I suggest making photo copies of 100's. If you are given back a counterfeit, just show the provider the photo copy of what bills you actually gave them & state emphatically "This is not one of the bills I gave you for exchange as the serial #'s are different!" You should report the incident so others are not scammed & fill out a complaint in the Libro de Reclamaciones.
To your continued health & wealth.
PS - thanks Alan for a very informative article.