Last year, I grew obedient plant (Physostegia virginia) for the first time. It was given to me by a friend whose obedient plant re-seeded. This spring while weeding that flower bed, I discovered what I believed to be obedient plant seedlings that came up from last year's plant.
Jack will tell you that I have been known to carefully tend weed seedlings far into the spring until it became obvious that they were not flower seedlings. So I decided I would try to find a picture on the internet of what an obedient plant looks like early on. I came across a document on the Natural Resources Conservation site that was extremely useful. It is a seedling identification guide, and it has great information (including pictures) of a lot of the re-seeding plants that can be tricky until you get familiar with them. Check it out here and possibly save yourself from tending the weeds and pulling up your flowers.