This fallen oak leaf is covered in galls.
They look like they’re caused by Neuroterus quercusbaccarum, which has injected eggs and produced Common Spangle Galls. They are a chemically-induced distortion in the oak leaf by a tiny Cynipid Wasp. They have a fascinating two-stage lifecycle with asexual and sexual generations every year and produce two different types of gall at different times of the year.
I need to up my macro game if I’m going to keep seeing them. The more you look, the more amazing they are.