I wanted to post this topic as a way for anyone going to Buenos Aires that will be using taxis to get around to be informed. Not to frighten or scare you off from using them.
My wife and I were down there last week and we used taxis pretty much the whole time. They were cheap and plentiful. But coming home one night from a Tango show we had a problem. We tried to pay our taxi with a 50 peso bill and he said it was false. Then I pulled another 50 bill and gave it to him and he said it was also fake. I couldn't believe what was going on. I wanted to pay the guy so I told him to take me to an ATM (Bancelo) so I can get another bill and when i gave him the 100 peso bill he also said it was fake!! Wow. As he was taking us back to the hotel there was an HSBC and I told him to pull over so I can take out another bill. After looking at the supposed fakes everytime he handed it back to me, I started taking note of the differences mainly on how they felt I knew immediately the one I took out of HSBC atm was the real deal and even told him. Then he said yes...this one is real. Gave me the change ( only to find out that the 50 peso he gave me back was also fake!) As we walked into our hotel we explained to our hotel front desk attendent on what had just happend and verfied that the bills we had were all fake. We still can't pinpoint where we received them from. From the taxi driver? From the tango show? From the ATM!?!??! Until now we still aren't sure. But it left a very bad taste being it was our last night in Buenos Aires. Everything had gone so well.
But you can inform yourself on how to prevent this from happening to you. I had no idea this was an issue or how rampant it was. It wasn't until afterward that night while researching on the internet that it does happen especially if your a tourist since obviously we are not familiar with scams as such since we don't live there.
Ask your hotel to give you a quick run down on how to tell the difference from a fake to a real bill ( I was so lucky be able to have an example of each : / ) Check the reflection of the number amount to make sure its reflective and changes color and it doesn't look out of line. The reflection of the preson in the bill when held up to the light shouldn't look to dark, you should be able to see the single strands of hair on the person on the bill, and feel of the bill shouldn't be to smooth. I'm sure there are countless other ways just. You could also look at the last 3 or 4 numbers on the bill before handing the bill over to a taxi or whoever. So if they come back saying its fake you tell them that wasn't the bill you gave them. Those are just a couple examples to help prevent you from being scammed and its a fact that as a tourist you are a better target.
Again this is just so your informed about that but this in no way should prevent you from not having a good time in Buenos Aires. Besides that little incident we ended up enjoying our time in Buenos Aires. It's a beautiful city with very nice people.