With the spring thaw this year, I discovered crop circles in the back yard!
But rather than blaming aliens or the paranormal, I blamed my own sloth.
Late last fall, we raked our mass of leaves into one mountain and one smaller molehill. It was dark by the time we finished, so we said we’d move it to the curb the next day for pickup. Then it rained, rained and rained some more. Weeks went by. Critters moved into the pile, which seemed to shrink under the accumulated weight of the rain. It snowed. And finally, one day in late November, we scraped the rotting mess up to the street to be sucked up by the giant leaf vacuum.
This spring, as the grass began to come back to life, the outline of the leaf pile still remained yellow and dead. In the past couple weeks, some life is returning, but it’s mostly dandelions.
Next fall, I pledge to promptly remove all leaf piles, no matter how dark it is!
Adventures of the Former Elginista 
I made the same mistake early on in my house-owning days. I made a big pile of leaves in the fall, which got wet before I could bag them. Thinking, “that’s basically a compost pile, right?” I left them till spring. Come spring, I not only had a big pile of rotten leaves to (still) bag, it also left a huge dead spot in the middle of the front yard.