Fake value.

False currency is common enough here that every store clerk will carefully examine the 100-peso bills for authenticity.  I do it myself, in my job, so it has become quite easy for me to identify a fake.

I've never had a fake US bill, though it seems to me that if you get caught trying to spend one at home you will likely get detained and thoroughly questioned -- at least. Here, you are simply asked to put that fakey back in your wallet and produce a real bill.

The other day at the bar, an American couple handed me a 100-peso bill to pay their bill -- clearly a fake. I told them so, and the guy handed me another as Brent explained how to tell the difference. The second bill was also a fake. The poor guy was getting flustered, and said with some shock in his voice that he got those bills from his bank back in the States. He paid with a credit card, and wondered if he would be able to take them back to the bank -- I told him to try, and my fingers are crossed for him. 

To be fair, a good fake bill looks a lot like the real thing.  Brent ended up with one in his wallet last week.  The difference is in some subtle details and the feel of the paper.

Of course, a bad fake bill is downright laughable. The girls at the bar were left a fake 20 as a tip -- and the bill´s design had been printed crooked on the paper.