If you aren’t careful, you might ruin an exposure even though outwardly everything was done correctly.
If light gets in the viewfinder, your exposure can be affected because this can confuse the camera’s light meter. The worst case is when the direction of the light is pointed directly at the back of the camera (sun at your back, for example). Experiment with this. Shoot normally (looking through the viewfinder) with the light source behind you. Now repeat this except step to the side so the viewfinder is exposed to the light. Examine the two histograms. Depending on the strength of the light your exposure can be affected by a stop or more – or not at all.
Some cameras come with a viewfinder cover for this situation. If you are within reach of your camera it’s easier to cover the viewfinder with your hand than it is to attach the cover.
Almost never a problem for hand held shots – but use your tripod anyway. Never a problem when using Manual shooting mode since the meter isn’t involved in manual exposures.
This is a real potential problem for anyone using Live View on a tripod with a current Nikon camera. You are unlikely to have your eye to the viewfinder (since it’s black in Live View). However, the mirror which is up during the Live View focusing step, drops back down before going up again during the image capture stage. This is when the light readings are made and when your viewfinder is likely to be uncovered. Beware.