Invasion of the lambs ears.
A couple of months ago I ripped out a load of Lambs' Ears (Stachys) because they were taking over and smothering other plants. For a variety of reasons I hadn't been into that part of the garden since. Down there today, and the entire area I had cleared is covered with Stachys seedlings. Jeez, they are invasive. I'll have to keep a close eye on the area from now on.