I didn't think counterfeiting was a big problem, but considering I saw these two posters on buses on the same day, I wonder if perhaps it is.
They both say that to safely use money, first confirm it is genuine, and never to use counterfeit bills. The second poster has more visible details (as if taking a photo of text inside a moving bus is an easy thing!) which say things like:
When receiving money, confirm whether or not it is counterfeit.
Confirm more than twice that it has the counterfeit preventative features.
Exchange money in a bright place.
Examine it. Shine a light on it. Tilt it.
So now you know...