It's so much easier to be a critic than to be a force for knowledge and goodness. Plus it's fun to point out others' mistakes while revealing none of your own. In that spirit, I offer two things you should strive NEVER to do in the garden.
First up is crappy topiary that looks like tuna cans then we talk about dangerous and strangely shaped trees.
I can't recall if I took this photo or not. It is representative of many gardens in the Los Angeles area. This gardener apparently has only one tool - a hedge shear. A level or plumb bob would have helped this look less crappy and less like it's sliding downhill. It still would have made my don't do this list, but at least it could have been level. Look at the hedge-shaped plant against the house. That caved-in look at the bottom is a problem that is addressed with proper pruning technique, applied over time. Now it's just a mess. What are they trying to hide up there anyway? The siding?
Second up is planting trees where they don't belong. I don't think that I took this photo, but it perfectly illustrates the problem with planting inappropriately sized trees. If a tree says it grows to 50' tall, why doesn't the home owner LOOK UP and see what it's going to run into? That's a first step. A second step is to think about how wide it's going to be. A third step would be to look at what it will shade. But mistakes happen - either yours or someone else's. So if you happen to own a tree that's ill suited for its location then you ought to TAKE IT OUT instead of letting the utility company do your pruning for you. If a tree is old and large it is NOT valuable by definition. Value comes from its benefit and if it's the wrong tree in the wrong place then it's the opposite of valuable. And then you end up with something that looks like a pickle or a giant 60s-era chair or an enormous jelly bean suppository. Where's the main trunk go up in the tree? Does it dog leg to the left? That doesn't look structurally sound. Oh, your house is underneath that cantilevered growth too? Too bad. You lose.