Many things marked with the Prop 65 warning do not actually contain anything carcinogenic (or rather, since everything is carcinogenic to some degree, they contain no more than trace amounts.) Legally, if a business has a Prop 65 warning, they're safe; if they don't, they're not.
The only teeth Prop 65 has against excessive warnings is that it makes consumers like me nervous, so it's better to not have it. If it were a little more sane -- like being based on actual exposure from reasonably expected usage -- then I think I'd like it, even if the list of chemicals is still a bit overzealous.
The system is too heavily tilted in favor of exposing people to nasty crap, and I appreciate having something that pushes against it. It's a hard thing to do well, and legislating via ballot initiative is a blunt instrument. So the best we generally end up with is stuff like Prop 65, which does some good, some harm.
What's the mechanism that the proposition puts in place? What makes an article be marked by the warning? Does the vendor have to report on the compounds their product contains? Or is there a state-conducted examination?
Pretty risky, just waiting for the wrong lawyer to notice.