I have been back and forth with the construction company about them finishing a job and fixing the patio cover. They were also supposed to install 2 ceiling fans and only installed one. They filed a lien and the amount is not the correct amount, the over charged on the lien.
Generally, no - a CA contractor shouldn't file their mechanics lien until the job is finished. Granted, if the project stops for some reason other than completion, it's possible that a valid lien might be filed.
Regarding the lien amount, California is pretty strict. In California, a claimant's lien must be for the lesser of (1) the reasonable value of the work provided, or (2) the agreed upon price of the work. If the claimant filed a lien for more than the permissible amount, the lien may well be invalid and unenforceable - and an owner should be able to dispute the lien and either have the lien tossed aside or at least reduced to the proper amount.
This article should be really useful here: A Mechanics Lien Was Filed on My Property – What Do I Do Now? And, for more information on the rules surrounding CA mechanics lien claims: California Mechanics Lien Rules & FAQs.