No photos – where can't you shoot in the big wide world?
When you head out into the world you might assume that all that you see is fair game for your digital image library, but that's not true. Haul out a camera at an airport security checkpoint, baggage hall or bag screener or point it at individual security personnel and you're begging for a slap-down. The layout of airports, how they're staffed and how they operate are all interesting facts to those who seek to do us harm, and you're unlikely to get away with it.
Some museums and galleries ban photography outright but most big-name institutions allow hand-held photography with the camera flash off. In mosques and temples, ask permission. Military aircraft, military storage facilities, naval bases, army bases and military personnel are all sensitive subjects, and anyone with a camera showing too much curiosity is suspect.
Photographing government buildings might be forbidden, but that may not be obvious until a security person appears in your viewfinder. In India, prominent warnings in government buildings forbid photography inside.
Some countries class railway stations, bridges and tunnels as military infrastructure since they're used to transport troops and hardware. North Korea took offence when photographer Eric Lafforgue posted a series of real-life images of the hermit state, including one of a subway tunnel, and banned him from future visits. Russia feels the same about the tunnels on the Moscow metro while India doesn't like you photographing bridges.