3 (a) That's right. Where the conduct that allegedly caused harm consists of a failure by the defendant to warn the plaintiff of danger, the plaintiff must prove that they would have taken notice of the warning and avoided the danger. This is because the law does not attach liability to a failure to do something that would not have been effective in preventing the harm. The same principle applies in cases where the defendant's omission consists of a failure to provide protective equipment.

In the present case, to hold B liable it would have to be shown, on a balance of probabilities, that A would not have eaten the substance if the jar had been labelled as containing poison.