I used to work at a natural gas utility, and heard that this actually happened. During a strike the normal workers were picketing, so the construction crew was made up of foremen and supervisors. They were installing a gas line for a new house in the old part of town where both gas lines and water mains were cast iron. When they dug down they came across both pipes and after years underground they looked identical. Normally the water pipe will have condensation on it. So they attached the "No-Blow-Tee" to the driest pipe and tapped it. The fitting allows them to drill into a live pipe without leaking any gas. Hooked it all up and turned on the gas.

The homeowner came screaming out the house, as water was squirting out the cookstove, furnace and water heater. They had tapped the water main not the gas main. Since this was all done by management, the union workers never let them hear the end of it, as the company had to replace all the gas equipment, and pay to repair the damages from the water.