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Argemone sp.
Looks like a thistle and has a flower like a poppy. It was an unexpected spotting, never knew of this plant.
Looks a lot like Argemone subfusiformis (native to the region), but it's supposed to be rare, so it could be a Argemone mexicana, wich is supposed to be a common weed, and this was found in a patch of California Poppy.