We dug through an 1 -1/4" gas main because their map had it 5' off from where it was. Luckily, the ditchwitch operator had the presence of mind to realize what the sand puffs were, turn off the machine, then run. Mapping accuracy matters.
This happens a lot in Florida. The problem is, water level near the costal cities is so close to the surface, buried lines often move anywhere from 1-3 feet from the original documented location. Verizon was rolling out Fiber to the Premises in Tampa, and finally after so many water/sewer main breaks, the state/county/municipality blocked them from using equipment to dig.. they had to start digging by shovel. That really put a damper on the FiOS deployment. Related article: http://www.heraldtribune.com/apps/pbcs.dll/article?AID=/2007...
A few years ago the city was running fiber for their internal network. They were directional boring under the street in the oldest part of town (circa 1880s mining town). They drilled right through a clay sewer pipe that was't on any maps and, since it was clay, didn't show up during the locate.
They ended up having to dig up the street anyway to fix it.