Multiple things lead to the conclusion
In my very first full time job, the offices were repurposed chicken sheds (don't ask me how they ever got the smell out - it was done before they were sold to my boss.)
We were having a problem with the water drain in the tarmac between the U shape of single storey wooden buildings.
Being very fit, and having access to a pick axe and shovel, the owner (my direct boss) asked me to have a quick look at the pipes.
I did the usual due diligence with him and got the site plans out. The mains feed was along the North side of the building on the North side of the tarmac. I could see that the drain was at least 30cm to water level and so the drain was presumably at this depth (otherwise the trap would not work).
And so I swung the pick axe just once and saw sparks!
Very carefully I investigated, with the owner standing a few feet out of the way.
It turned out that:
1) the building on the South side of the U shape had a different mains feed (that joined the other feed before the meter).
2) that mains feed had been placed in the water drainage pipe's trench above the pipe.
3) the building on the South side had been built later and the original North side drawings only partially updated.
4) the delivery lorries we had in perhaps once a year, followed the same route as the agricultural vehicles that originally serviced the chicken sheds - no other vehicle was allowed down there. It turns out this was the only part of the tarmac with any form of hardcore underneath. The pick axe had gone into tarmac that was 2cm deep over hard packed soil over the trench.
So multiple things that shouldn't have happened did and resulted in a lot of cost for:
rewiring (to standard)
replacing the old clay drainage pipe that was partially collapsed (under the old tractor route), full of soil and chicken shed litter
a new layer of tarmac over the whole area to a reasonable depth
newly corrected plans