Installing recessed downlights everywhere. This is one of the most common errors that lighting design professionals see. Builder spec versions of can lights can be very inexpensive, and people often assume that laying them out in a regular grid gets light everywhere. Unfortunately, this is not so. The optics of inexpensive can lights often allow only slightly more than half the lumen output of the lamp to escape. In addition, can lights, unless they are adjustable or wall wash fixtures, typically don't put sufficient light on vertical surfaces, which is where the eye perceives light. With an array of cans, we might waste nearly half our watts and still have a space that feels like a cave because the walls are dark.
Forgetting task lights in the kitchen. There are many better ways to light the counter, and one of them is to use fluorescent (T5), xenon or LED task lights under the upper cabinets. If your kitchen design lacks upper cabinets over some work surfaces, don't worry. This is a situation where wall-mounted or ceiling recessed adjustable fixtures with the right lamp make all the difference. Adding several low voltage halogen fixtures with a narrow flood beam distribution and focusing them on the task area will do the trick. Remember to choose your fluorescent or LED color temperature wisely.
Using downlights over the vanity without adding lights on the side. Standing directly under a downlight, without any light at the sides of the face, creates exaggerated and unflattering shadows. In the bathroom, using a downlight over the sink is fine to accent the expensive polished nickel faucet you've specified, but it's insufficient for tasks like shaving, tweezing, and applying makeup. For this, we need light at the sides of the mirror at eye level to minimize shadows and provide even distribution. This can be achieved with sconces flanking the mirror.
Neglecting to control different types of light separately. For maximum efficiency and flexibility, each type of light should be controlled separately, and any incandescent or halogen light, or dimmable LED's should be dimmed. Controlling multiple sources can be achieved by the old school method of multiple light switches, but there are many more sophisticated ways to achieve control. From a simple programmable wallbox system for single room control with preset scenes, to wireless controls that generate their own power and can be reprogrammed from a laptop or phone, controlling the lighting yields energy savings combined with the right amount and type of light for different times and uses.