Most people don't measure before they buy. Very important is to measure your home BEFORE you go out and buy lights - that way you can determine the size and width of the light (and how high it should be off the floor if it is a pendant light).

A lot of people forget about dimmers, and realize only too late that they need dimmers and dimmable lights / globes.

Make sure that your light does not clash with your room. For example a Victorian room should not have an ultra-modern chandelier, but rather a classic one.

A common mistake is to only have one ceiling light in the bathroom. This will create unpleasant lighting with lots of shadows. Add a sconce or a vanity light for layering.

Ensure that your ceiling lights are well spaced and have a good beam angle. A very tight beam angle can produce very bad pools of bright light inside lots of dark shadow.

Most people think they can just add lights where ever, and they will work. WRONG WRONG WRONG! Lights are critical and if you want to get it right, use an interior designer that works with lighting designs.

One of the most common mistakes, particularly in modern buildings, is to use recessed downlight is every room. This creates a very bad effect beacuse often the walls are not properly lit.

A big mistake is to forget under cabinet lighting in kitchens, which will create really bad shadows on benches and stove tops. 

As an interior designer, I often see the problem where rooms have downlights installed and nothing else. This is very non flattering because it creates ugly shadows on faces.

Biggest mistake is to forget dimmers!

A common mistake is to forget the SIZE of the room - ie that large rooms require mofre lights and conversely small rooms should need less lights.

Another mistake that people make is not to understand colour termperature and then installing ugly cool white to daylight white lights in their living rooms.