San Juan, Costa Rica
Going from downtown to the trendy shopping and dining area is dirt cheap (couple dollars for 2). The return however is 10-20x higher. It seems that public cabs take you there but only private cabs bring you back.

Prague, Czech Republic
Do negotiate the fare before starting -- I was going intra-Praha 6 and knew my destination was close, just not exactly where or how to get there.

I gave the driver the name of the place and the address, he gave me an absurd quote.
Me: How far is it?
Him: 20-30 minutes?
Me: Bull -- it's 5-10 minutes.
Him: Oh that whatever the street was called. There are 2 locations on 2 different streets with the same name