I learned after owning several houses never to assume that whoever did work on it at any point was sane, rational, reasonable, forward-thinking, accurate, hard-working, standards-compliant or at all even human.

I tested a wall with a cable finder once... nothing. I checked back and front of the wall visually ... nothing. I aligned all the local sockets and checked where wires would have had to run up or down... nothing anywhere near. I drilled in the middle of a wall 4 feet from the only light switch where nothing was reading. I hit the cable. Yep, rather than go "straight up" into the ceiling to the light fitting, they took a right-angle 4 feet detour through several pieces of wood for no discernible reason.

I once pulled up a floorboard, looked down it, and put my hand inside to pull out a handful of some insulation in order to make a hole to the floor below. My hand came back holding a large core wire, bare ends, no covering, not even tape. Can't be live?! Surely not!? Fucking live. 32A 220V live circuit, just shoved randomly into some insulation without any protection whatsoever. Not associated with anything nearby, not even any nearby electrics!

I needed to move a thermostat. I saw a metal backbox on the wall inside a tiny cupboard under the stairs, with a wire going into it but nothing coming out or on the other side of the wall. The backbox was just balancing there on a tube that contained the cable, so I could get behind it, under it, around it... no outputs on it at all. Obviously some legacy cable to run something at some point.

Unscrewed the metal box... another cable, ends dangling loosely and precariously inside a METAL backbox. Guess what?... live. Rather than terminate it, rather than seal it off, rather than turn off the (redundant) circuit it was connected to... just shove the three live wires into a metal backbox and put the metal plate over the front, it'll be fine in that dark cupboard where you cannot possibly escape.

Toilet cisterns not fastened to the wall at all, just balancing on the back of the toilet.

An entire huge concrete water feature buried, still with intact pond liner, 2 feet deep in the garden (we were wondering why that spot never seemed to drain and was always wet). Rather than remove it, they just knocked it over into its pond liner, then poured more soil on top.

A small porch entrance that was literally balancing on some breeze blocks - hidden behind panelling underneath the now-sinking porch as they sank into the ground themselves. Had to knock the whole thing down and rebuild it, because it had sunk so far it had started to bring the front of the house down with it.

200+ dead rabbit bodies in the garden lawn. Previous owner had been a rabbit breeder, they all died of myxomatosis, so he just buried them and laid a shallow lawn over the top. That grass went absolutely batshit tall with the fertile soil of them all, but you couldn't dig a thing because of all the bodies and bones.

A radiator literally INSIDE a wall. Rather than remove it, they just boxed it off within a new wall. Still connected, with water pipes and heat coming off it.

Power sockets underneath a floor, running extension leads also under the floor that powered things that had been in the place 30+ years. Literally just shoved an extension lead underneath a wooden floor and plugged everything into a hidden socket underneath the floor (no official access, I just pulled a cable and an extension lead came with it... pulled the lead and eventually an open-backed live socket came with it). Worked fine right up until it didn't.

Once laid a laminate floor and the first step is to put the corkboard lining down across the whole floor, then put some small strips of it as cushioned edging (to act as an expansion gap around the walls) and then you put the first plank in place and just tap it snug against the cushioned edge. Done it a dozen times. Watched my dad lay the cushion edge, then lay the first row of planks carefully against it, tap them in place, no problem. Did the whole row. Moved onto the second row and because it was click-together laminate we wanted to make it secure. Dad had made a little "template" to fit snug on a board edge so you can tap it with a hammer snug against the first row without damaging the "keyed" part of the board, and so on.

It wouldn't snap together, so the tapping had to get slightly more insistent. Still wouldn't snap together. Rejigged it all, tried another plank, made sure there were no obstructions, etc. Give it a decent - but not damaging - thump. The entire cushion and the first row of laid flooring punched straight under the wall and slid four feet into the next room. The entire wall was... floating from the ceiling. Not attached to the ground anywhere. First person who fell onto that wall would have brought the whole thing down on their head.

Assume nothing. Buy a snake cam (they are dirt cheap on Amazon and can run off your smartphone). Drill in a "safe spot" (no such thing!), snake it, observe where you want to drill through. Drill slowly, carefully, and make sure your drill is earthed properly. Stop at any obstruction, restriction or resistance.

Turn off the power when you do it (good luck with relying on that AND powering a tool!) and then check the hole carefully for anything you might have cut through (again, snake cam).

Assume nothing.

Question everything.

Try not to die.