It’s important to make mistakes and use those mistakes as learning opportunities. One mistake is not starting planting in springtime. Feel free to make mistakes with small annual plants, but try not to make mistakes with trees or shrubs, and do your research when planting longer-lived perennials. Even smaller perennials, you can make mistakes with because you can move them around afterward. Another mistake I see is people planting sun-loving plants in shade, or planting shade-loving plants in the sun. It's also important to think about drainage. Some plants need better-drained soil than others.