And of course there are marker posts that say "buried fiber optic cable do not dig".
Most cheap hired labor doesn't realize you shouldn't dig up the market posts and throw them in the dumpster on construction sites. Add a little bit of rain and suddenly the contractor showing up with an excavator that was told everything was properly marked, and can see markings in other places but not where they are digging, and disaster occurs.
Also soils aren't static, I've been on sites where the midpoint of a buried telecommunications cable had drifted over 8 feet from where the markers were a few hundred yards apart. Just looking at the markers and assuming a straight line isn't a safe bet. Looking at the fence rows to the left and right of the cable and you could see a matching bow in the fence rows.
Reps from the local telco showed up to one of the meetings to tell everyone that they didn't know where their copper lines were buried and that they were unsuccessful at locating them outside at all. They knew where both ends were but the middle was a mystery to them.
This statement made the architect's face contort into strange shapes. I joked that they call 811; some people thought that was rather hilarious, others much less so.
Anyway, the building wasn't in service yet and the telco said they'd fix it if we managed to naff them up, so away we dug. No telco cabling was damaged in this process. (We did dig up some plastic drainage tile but that was a simple farm store fix.)