This HGTV list includes a common mistake--Planting trees and shrubs that are too big for the space! And never ever plant trees too close to the house. (There goes the foundation, the septic system, the sewer pipes, ...) Don't plant too many of the same thing. This doesn't apply to a bed of tulips or roses, nor to our collection of crape myrtles. (We have lots of other trees and shrubs and flowers.) They point out that this gives you limited flowering; you might want to have colorful blooms for the whole season.
And if you're building or remodeling, don't put your plants where workers and vehicles will be busy. (When we built, we started landscaping the perimeter. 'Course we had five acres to fill so the contractors had plenty of room. But remember to protect trees and plants with yellow tape.) And watch out for ivy taking over your house. For more info and scary photos, go to their link: 7 Top Landscaping Don'ts : Landscaping : Home & Garden Television
I was hoping they'd mention pollarding, also known as crape murder when inflicted on crape myrtles. You might think that would go under planting trees too big for the space, but some people prune the hell out of them even if they've got the room to grow! And it's better to plant smaller shrubs than spend the rest of your life pruning a hedge or paying someone to do it.