Don't we all just love those good old succulents? How about this rather pretty little plant, Carpobrotus edulis, also known as Highway Iceplant? It's been planted generously round many parts of California since the early 1900's, up until citizens in the 1970's finally spoke up and said it was a really bad idea.
Unfortunately Carpobrotus edulis is a really bad neighbor, moving in from lands afar and then taking over. It's a coloniser, yah know how that is, right? Those folks come with beads (or in this case seeds!) in hand, and the next thing you know there's no room for the old folks to roam. You'll find large mats, like the one above at lovely local beach sites, along highways, and old railroad tracks - where it was planted to keep our soils from rolling down hill. It did do that, but it also spread, spread, spread. And it just keeps on spreading.
You can read more about highway ice plant invasion problems at this Invasive Plants of California Wildlands site.