Pinhole leaks in wastewater lines near rivers which could lead to a blowout, buckling in pipes due to blockages that haven't been cleared, water lines that weren't buried properly and are rapidly deteriorating due to exposure. I deal with municipal and city lines, mostly in the middle of nowhere where nobody really checks on them. Had one that hadn't been checked in over a decade, but was listed as "in good order" the previous year despite having a noticeable long term leak. I'm still pretty new to the craft, but the older guys on my crew aren't surprised when we come across something that says it's good, but is obviously not.