Um, I lied, for the sake of a consistent title.
This blob in the photo is yellow, yes, but it is a slime mold, not a mushroom.
Slime molds consists of undifferentiated cells that form a mass called plasmodium. This one is called scrambled egg slime (Fuligo septica):
Olympus XZ-1, 55mm, ISO100, f5.6, 1/30s
A bit scary: these masses of cells cooperate and actually move as a whole when looking for food!
Fortunately, they move very slowly, so there is no need to start running immediately when you see one of these. I don't know what would happen if you sat close to one for a week, though.