Some people experience a very strange artifact in their picture when shooting with a (very) long exposure. Corners of the picture become purple or bluish, or pinkish. This phenomenon is normal. It is a form of noise influenced by the temperature. The sensor is getting hotter with time, and the problem gets worse. How can you limit the effect. On most camera's, there is an option to activate noise reduction for long exposure. Just select that option. The camera will then expose a reference picture and subtract it from your shot. The result will be quite acceptable. Since the issue is made worse with high temperatures and using long exposures, you can slightly influence the behavior by shooting when it is cold, and by waiting between shots long enough to allow the sensor to cool down.
Some more detailed and scientific explanations can be found here:
http://www.photo.net/learn/dark_noise/