The most common of scams is the cab driver quoting a steep price for a short distance.
Once I arrived by bus to Bratislava, I took a cab to my hostel. Since my internet wasn’t working I did not know how far the hostel was. The driver stated that he would charge by the meter. Once I arrived at the hostel (which was a short trip), the driver demanded 30 euros.
The funny part is that I traveled all the way from Budapest to Bratislava for 2 euros (by bus) only. I have taken taxis in Europe before, so I knew he was surely overcharging me. But, there was no way to negotiate, I ended up having to pay 30 euros.
I have been approached by numerous such cab drivers across Europe and Asia. I have noticed such incidents take place outside of airports, bus terminals, and train stations when the drivers know the passengers are tourists.