Often I see people not changing their settings to reflect the ambient light. This is crucial if you want to produce a realist photograph of what you're looking at!
This image was taken in less than optimal conditions, it was shot at the end of a safari, minutes before it was completely dark. The only way I could get the shot was by setting the exposure compensation to -1.3 EV. Otherwise my camera would be trying to make it lighter than it actually was!
So remember, in dark situations, you need to underexposed your image and in bright situations you need to overexpose it! :)