liability for negligence

A crucial factor in establishing liability for a slip and fall injury is showing that the defendant had notice of the hazardous condition. Without such notice, no liability will attach. The plaintiff may satisfy this burden by proving actual notice, or by showing constructive notice. 

Actual notice of a particular hazard may also be established through third persons who testify that they warned the defendant directly of the dangerous condition prior to the injury-causing accident. 

http://www.bpslawyers.com/Articles/Actual-Notice-of-Hazardous-Conditions.shtml