Many years ago I needed to get an RS232 connection from the basement to the 1st floor of the office I was working in. At the time there was a mass of twisted pair cables that ran around the building, used to hook all the phones up. I was able to isolate a couple of unused pairs and hooked up the RX, TX and GND. The hack worked a charm and the data flowed.
Fast forward 3 months. The workers came into the office one Monday morning to find that the phone system was not working. Going on the fact that I was the last person "messing" with the system, the boss decided that my hack was responsible for the failure (bear in mind it had been working fine for 3 months). So I set about unwiring my makeshift RS232 cable. And surprise, surprise, it failed to make any difference. The non-working phone system remained non-working. So finally the boss had to bite the bullet and call in a proper phone engineer to sort the problem out. It took another day but the phones started working again. And nothing more was said.
But here's the thing, I knew the phone engineer outside of the office and we ended up bumping into each other later that year and decided to go for a pint. I asked him about the phone job and he laughed- Apparently, the boss had decided to change the carpet in his office. So he decided to do it on the cheap and he and his father-in-law had come in on the Saturday, pulled out the old carpet and replaced it. The fault with the phones? They'd used a stapler to fix the carpet and managed to put a staple through the phone lines. Not once, but many times.