This trick has to be even more widespread than the "almost free" lie, and it's even more aggravating. You go to the ATMs in Mexico, and you usually get a wad of 500-peso notes, sometimes with a few 200s thrown in. Then you try to use these notes to pay for something — dinner in a restaurant, for example. 9 times out of 10, they'll be rejected, with the excuse "no tengo cambio" (lit: "I haven't got change"). ¿No tiene cambio? Why the hell not!
I've encountered "no tengo cambio" in pretty much every place that you'd expect would and should have change. Bars, restaurants, hostels, tour agencies, bus ticket offices, museums, and corner shops, to name a few. And of course markets (which is more understandable). Where is all the change in Mexico? And how do they honestly expect you to have (almost) exact change in pretty much every place you go?
It's ridiculous, and in many cases I also suspect that it's fraudulent. The change situation really needs to change.