Airports, flying and X-ray machines
Airport X-ray machines can damage film and the effect can show up as fogging, strange bands, or a general loss of contrast. The stronger CT scanners that are becoming more common at airports are especially worth avoiding. I always recommend asking for a hand-check of both your film and the camera if there is film inside it. I have found that people at security are usually used to this, but it still helps to keep the film easy to access and ask nicely.
Never put your film, or your film camera (with film inside), in checked luggage. Checked bags go through stronger scanners that completely wreck the film. I learned that the hard way.