Maybe you’re out in a pub or restaurant. The light looks fine, and things even look good through the camera’s viewfinder. It’s still possible to take a bum shot. In mid-light conditions like this, you can still get red-eye unless your camera compensates (the flashing before the shot) and you can create a shadow if you stand between the light source and your subjects.
Some cameras have red-eye sensors on them to get of the problem before it happens. But to avoid it all together, try putting the light on before taking the picture and asking people to look to the side of the camera slightly, not right into it.
