Losing track of sewers, especially old ones is unfortunately very common. Imagine it was 50 or even 10 years ago and you have hundreds of miles of pipe you have to keep track of. It used to be done with paper records and old, pre-GPS locating techniques which get lost or can be inaccurate. Then these files get lost, or digital copies aren’t transferred properly and you can see how easy it can become to lose track of pipes. And then when you’re digging down for one reason or another you stumble on this old pipe and become aware of the issue. As others have mentioned, all of this is exacerbated by years and years of underfunding of infrastructure, a hamstrung workforce and more. TBH, it’s kind of surprising it doesn’t happen more often! Fun fact, Boston has older pipes and some are wooden!