I had the bad experience in Kuta, Bali Island.  Just did a quick research online and found it's actually kind of common in the country.

One USD is worth about 13,000 Rupiah, The Indonesian currency.  That means their bank notes bear very high numbers, like 100000, 50000, etc.  They don't use commas to group the zeros, so one big challenge for foreigners is to count the zeros on a paper money.

It's not uncommon that money exchangers purposely use lower value bank notes, e.g. 20,000 Rupiahs other than 100,000 Rupiahs, to exchange your USD.  That means there are lots of bank notes to count.  Counting money is the part they do trick on you. 

When I went to exchange 200 USD in a small store in Kuta, the guy took out quite a few stocks of Rupiah money.  He counted in front of me, but he had very quick hands!  He took away the money in front of my eyes without my notice.  When I went back to my hotel room, I found there is a short of half million Rupiah.  That's $50 USD, a quarter of the amount I exchanged.

When you go to Bali Island, stay away from Kuta.  Also only exchange money in bank offices or dedicated money exchange shops, which would provide a transaction receipt.  Always count it with your own hands.

Basically same precaution applies to everywhere, especially developing countries.

Wish you a great trip!