I was glad to see some of these common mistakes listed in the December 2007 issue of The Urban Horticulturer (U of Florida Extension/Polk County newsletter):
Over planting/planting too closely in beds. Find out how big those plants are going to get! Forget instant gratification.
Planting too close to the house: I've seen a lot of trees too close to the house. They can take out the foundation (where do you think those roots are going to go?!) and rot the siding (moisture and mold buildup.) There should be at least 12 inches between the plant and the house. Find out how wide they get. (I never thought I'd have to trim bushes by the house, but a landscaper refused to listen to me. I rarely use landscapers, btw, but I splurged. We had to remove most of those bushes; they were too close to each other also.)
Lawns cluttered with trees and bushes: Make large muched beds; it cuts down on mowing and makes the plants happier; and I don't have to dig a hole in the lawn to plant something new. I've gradually enlarged smaller beds to include nearby trees, bushes, and flowers. This was done over the years and required loads and loads of mulch; I now get free mulch from the electric company. And group like plants or plants by color. We have white beds and pink beds, etc.
They didn't mention myrtle murder though. This is pollarding (lopping off the tops of crape myrtles). Some crape myrtles were planted too close to something and had to be pruned; but too many people have been brainwashed into thinking that that is how crape myrtles are supposed to look!