A cross-connection is a connection of a potable water system to a non-potable system or a system of questionable water quality.

cross connection correct way example
Where backflow occurs and cross-connections are present you have all of the necessary elements for contamination of the plumbing system and subsequently contamination of the public water system.
cross connection wrong way example

Example of Contamination

For example, suppose an automatic lawn sprinkler system is spraying a lawn when all of a sudden a backsiphonage occurs due to flushing a hydrant for maintenance, or water rushing from a broken water main.

The resulting backflow from the lawn sprinkler system will flow into the plumbing system and then into the water distribution system. As the water backflows it can suck contamination into the lines through the sprinkler heads such as insects, pesticides, herbicides, fungicides, fertilizer, worms, and other contaminants.

Once in the distribution lines the contamination could go anywhere in the public water system.
picture of truck and hydrant flushing