Legal Question in Construction Law in California

I don't bother to keep up on building codes by memory, but the issue would seem to be whether the applicable building codes required you to replace the supply line when you replaced the toilet. If so, you're probably liable. If not, there are two follow up questions - 1. was the supply line in such a condition that you knew or should have known it would fail within a reasonably short period of time; and 2. did anything you did or failed to do in the course of the installation contribute to the failure of the line? If yes to either of those questions, you are probably liable. If you answer no to all three questions AND have at least some proof of your answer, you should not be held liable. In any case, if you have liability insurance you should tender a claim to them. They will sort it out and most likely provide an attorney for you if it goes that far.