A cafe was receiving complaints about food poisoning coming from a few meals containing a

side salad with lettuce. These complaints appeared to be coming in groups every few weeks.

The restaurant narrowed the food poisoning to the salad, which contained lettuce that was washed in tap water. A month after this all happened, it was

discovered that there was a water leak in the main, near a construction site, and contamination

from the site was entering the water system. The contaminated water was then used

to wash the lettuce, causing food poisoning-like symptoms in the customers. This is an

example of: