Only in Cambodia

Are you sure that while creating a scene, you didn't distract the cashier and she simply made a mistake?  It seems plausible.  You might want to give the benefit of the doubt.  Next time, count your money before you walk away, it's YOUR money at stake.

Actually, I wasn't creating a scene.  There was a time when things like this would really piss me off. In the past I'd make a scene when someone cut in front of me, and both the cutter and the cashier would just look at me like I was crazy, not understanding why I was making such a big deal out of it.

It used to be a lot worse ten years ago  when people seemed to have no concept of taking turns whatsoever. But I've come to realize that I had been raised in a different culture, and it's not fair to expect Cambodian locals to share the same mentality. I guess getting cut in line in Cambodia is sort of like talking about the weather-- just like you're not supposed to get offended when discussing the weather with other people, you're not supposed to get offended when someone casually cut in front of you at a supermarket after having waited 15 minutes for your turn.

At any rate, I stayed calm and did not raise my voice at all when I complained to the cashier.  In fact, I didn't even take the incident personally and simply wanted to educate the cashier to prevent similar behaviors in the future.  As someone who has a tendency to trust and see good in people (often to a fault), I'm willing to consider the possibility that the shortchange was just an honest mistake. Interestingly, vendors here never make the mistake of giving me excess change.

I recently had the same problem. ***

Concerning 100-dollar bills, I almost always change them into 20 notes of 20 000 riel (=5 dollars) at the exchange booths outside the old market. So, one note is pretty much 5 dollars. I avoid spending the 100-dollar bills directly. Otherwise, I will have to thoroughly account my change, and I may not really be interested in doing that, exactly then. I have never been short-changed by any of the exchange booths outside the old market. They seem to be very reliable, which is something you cannot necessarily say about local supermarket staff. But then again, how would you be, if you got paid with small bags of peanuts? That is obviously what explains the monkey business. They do not have much to lose, do they? Since they are able to count to ten making less than three mistakes, they are in high demand everywhere for their otherwise rare and almost unfindable skills. So, the next day they are hired again anyway and can start ripping off elsewhere!

Saying that the cashier cheated you is a blatant insult and based on only your assumption.

You made two mistakes:

1. You should have told the cashier to not serve the cut-in-front customer, they usually (at least in my case) obey to that, leaving the "cutter" looking angry at me, which is not important to me.

2. You did not check your change! That is your fault and the only one you should blame for this is you!
Everybody knows that a cashier can make a mistake. But it is up to every customer to check the change and if something wrong, claim it immediately. I must say in the supermarkets I come the cashiers always count twice, then, as the register shows how much change I get it is easy for me to check if it is correct. I have not found one occasion where the change was not correct. But mistakes happen.