You may have seen a product label which states that, “this product is known to cause cancer”…if used in California. (ha)
Proposition 65 is essentially a labeling rule. in 1986, California made a rule mandating anyone selling to California to label the product… if it had a serious health risk. This rule was above and beyond the OSHA reporting limit of 1%, like on a MSDS (henceforth called SDS, BTW).
The rule is actually good. It makes manufacturers tell you if there is anything hazardous in their product, or if they used anything hazardous when they made the product. (Side: If you haven’t heard, some manufacturers like to hide their ingredients, some say for proprietary reasons. Here’s an example)
More information about the rules is here. There is controversy over it’s usefulness (see here), but in this day in age, I believe you should be made aware of the information.
What usually happens is you find a SDS with no information about the product. Then, as you investigate, you find a “Proposition 65 warning”, indicating something about the product which, “may cause cancer”. So you ask, why does this happen? (more FAQ here) Well, the manufacturer used, or contains, something in their product that is hazardous. It can be a nice trail leading to chemicals which to sample for, or investigate.