It is the go-to defense in slip and fall cases, because it shifts blame from the property owner to the injured person. But an argument is not a verdict. Property owners have a duty to keep their premises reasonably safe, and whether a hazard was truly avoidable is a factual question — not a given.
California follows pure comparative fault, which means that even if you are found partly responsible, you can still recover, with your compensation reduced by your share of the blame. So even where the owner argues you should have seen the hazard, being assigned some fault does not end your claim — it affects the amount.
The way to answer the blame is with evidence: photos of the hazard, surveillance video, cleaning and inspection records showing the owner's notice, and witness accounts. Building that record is a core part of representing someone hurt in a fall.