Quite a few here on Rosa californica (I think that's the species).
Update: The rose has been id'd as Rosa spithamea
https://www.inaturalist.org/observations/47232120
3 out of 5 observations moved locations so far, Mt.Umunhum trail
So cool! I was like roses do not make fruit like this, what is this thing? It must be a gall or some other kind of parasite! 🖤🖤