Possible taxi scams if fare is agreed in advance?

The advice is given many times to not get into "illegal taxis", because they will scam you. My question is: what are the possible ways of scam, if you agree on the price and destination in advance (e.g. by showing it on a map)?

Well, I can imagine that taxi driver at the end says a higher price then agreed, but then what happens if you just give him the previously agreed price and walk away? In the answers of another related question it was mentioned that the taxi driver could deliberately "misunderstand" the location, but that would be covered by showing the map part.

For the sake of this question, let's consider only countries, where being kidnapped/robbed/killed by the taxi driver and such extreme outcomes are extremely rare. Say, Europe, US, and other countries considered "safe". (In other words, let's suppose you don't have to consider the risk that "the taxi driver will kidnap you".)

As answers I'm either expecting any theoretical possibility where you could be scammed, or even better concrete known instances of such scams (i.e. the tourist had to pay more, in spite of doing the above due diligence).

One particular example: I heard once (but not sure if it is true or not, since I can't find anything on Google about it), that a possible scam is agreeing on say "30 dollars" in advance. Then, at the end of the ride, it turns out that the taxist did not think about "US dollars", but "some other kind of dollars" (which of course are much more valuable then "US dollars"), and maybe he has even an accomplice to validate that ("Yes, of course, in this country we always refer to XXX dollars, not US dollars.") But as I said, I can't find anything about this, so maybe it does not exist at all.

Backstory: I was in a Balkan country, waiting for the bus to the airport, together with two locals (or at least they were from some other Balkan countries). A taxi rolled up, and offered to take the three of us to the airport for an extremely low price (basically what the bus ticket per person would have cost). I double checked the price I would have to pay with the taxi driver, before getting in, but was still expecting some bad surprise when we arrived. In the end, everything went fine though, he just asked for the price we had agreed on (and besides that I willingly gave him a tip), but at this point I'm wondering what was in it for him? (The total the three of us payed was less than half of the regular fare it would have cost to get to the airport.) Even though I had luck this time, I still think it was foolish from me to just get into the cab, and I would like to know what else could have gone wrong. (Btw.: I used the term "illegal taxi" at the beginning, this one seemed pretty legal though - had the sign at the top, taximeter, etc.)