Presumably it's this: https://en.wikipedia.org/wiki/1978_California_Proposition_13
> Proposition 13 (officially named the People's Initiative to Limit Property Taxation) is an amendment of the Constitution of California enacted during 1978, by means of the initiative process, to cap property taxes and limit property reassessments to when the property changes ownership, and to require a 2/3 majority for tax increases in the state legislature.
The insidious part is that commercial real estate in California is paying the same rates as they were when the building was built, despite the fact that the value of the building has increased many times since and the building "changing" ownership many times since.
The second insidious part is that because of the enhancements to the tax code over the years it's possible to continue to pay 1978 property tax rates in perpetuity, and even pass those rates on to your descendants. You can even transfer those super low rates to a new property in some cases.
New buyers get screwed because they will be paying at today's rates. So you could be paying 10x more property tax as your neighbor for the same city services.