We all learn by making mistakes, when we do the wrong thing and think, 'oops, won't do that again!' But the problem with making gardening mistakes is that you might be stuck with them for a long time, perhaps forever.
So here's one of mine: thinking that since I've never managed to grow mint particularly well in a pot I'd put some in my boggy spot in the front garden. Even though I knew it could be (is) a weed. And what do you know? It's taken off. In the space of six months it's become quite enormous and is sending new shoots out and rapidly colonising the front garden at an alarming rate. I used to think you could never have enough mint in the garden, but I might have to eat my words (literally).
So, fess up, what are some mistakes you've made in your patch?