The currency exchange trick
Covered in detail in our budget guide, but worth repeating: booths advertising "0% commission" near the main squares tend to bury their margin in a bad exchange rate instead. Use a bank-affiliated ATM (declining any "dynamic currency conversion" prompt) or a reputable, transparently-reviewed exchange office slightly off the main squares.
Restaurant bill padding
A small number of restaurants right on the busiest tourist streets have been known to add unordered items to the bill (an extra basket of bread, a "cover charge" not shown on the menu, or a drink swapped for a pricier one), banking on visitors not checking closely before paying. The fix is simple: check the printed menu prices before ordering anything placed on the table unprompted, and read the bill itself before paying.
Taxi overcharging
Unmarked taxis, or drivers who approach you directly in tourist areas or at the airport rather than working from an official rank, are the most likely to run up an inflated fare. Stick to official ranked taxis with visible pricing, or use a rideshare app, which fixes the price before you get in.
Petty theft in crowds
Pickpocketing is the closest thing Prague has to a real property-crime risk for visitors, concentrated in the densest tourist crowds — Charles Bridge, the Astronomical Clock at the top of the hour, and public transport at rush hour. A closed bag worn to the front and normal city awareness is all this really requires; it's not a reason to avoid crowded sights, just a reason to keep your bag zipped.
The general rule
Almost every scam on this list has the same fix: know roughly what something should cost, or what's actually printed on the menu, before you commit to paying for it. Prague doesn't require unusual vigilance beyond that — just the same ordinary care you'd take in any major, busy European capital.