If there are any mistakes to be made in plant placement, I think I've made them at least once.
Sometimes I have planted two or three together that are too similar and then none of them seems to show up in the garden, they all seem hidden and unnoticeable. I've learned to place some selfs between some of the busier colored ones, and that makes them both show up better. Some bold colors next to some subtle colors seems to make them both show up too. I'm experimenting with a bed I redid this last summer where I take a color from subtle to bold in a progression. i.e. from the very light orange with the next one being a bit bolder and the next one bolder still until I get to a very bright orange. Then a different color doing the same thing. Guess we'll see how that works in the future. I got it planted pretty late though, so I don't look for much this spring.