Every time I take a walk along the Discovery Trail in Long Beach (WA, not CA), I get a kick out of this sign. I know fully well what it was originally referring to, and yet I can't help but appreciate the utter smartassery of likely multiple people adding their commentary in the form of literal stickers. Technically, yeah, it's vandalism, but it's pretty low-level vandalism, and if the additional visual interest gets people's attention about the warning, and also makes them smile, that's a bonus.
Also, FWIW, these are the "stickers" the sign is supposed to be warning people about:
This is sandcarpet, Cardionema ramosissimum. It's a native plant that grows along the Pacific shoreline of North America from the northern end of Washington down to Baja California. It's pretty benign when green, but once summer hits and vegetation begins to dry out, the flowers turn into these horribly sharp, pointy little seeds that bury themselves into the hair or skin of any passing animal, human included. Unlike goat's heads (Tribulus terrestris), sandcarpet's spurs won't poke through shoe soles or bike tires, but if you track them indoors (or walk around barefoot outside) they will painfully pierce skin, hence the sign warning people not to tread on this particular plant.