Examples which may occur in your observation area(s) where flowers look very similar and where additional photos would be needed (as @thebeachcomber recommends):
- Coulter’s Matilija Poppy (Romneya coulteri)
- Hairy Matilija Poppy (Romneya trichocalyx)
- Flatbud Prickly Poppy (Argemone munita)
There’s also ones like Prickly Pears (Opuntia) and Cryptantha which are hard to ID to species-level and are only likely to get confirmed at Genus-level.