Long RS232 cabes
I have two stories about long-run RS232 cables.
The first was when I worked at a UK educational establishment. Pretty much everything was jury-rigged there, because they did not want to pay professionals to do things like laying cables. Anyway, my PDP11 was having some cables run down the hall to the lecturers offices, where their newly bought BBC Micro's were to act as terminals (and, in fact, were also connected to the floor Econet). The cables were laid, and loosely tied to a convenient support. Everything worked fine. One of the other technicians then decided ti tidy it up, and proceeded to tack it to whatever was available using staple gun instead of wire clips....
You can guess. Staple straight through the cable, shorting pins 2 and 3 (as we later found out). No immediate problems, because the BBC micro was not being used. But the PDP11 started getting slower and slower, and after about 4 minutes or so, crashed with a I/O buffer overrun error. Rebooted, and the system was fine for four minutes, and then crashed again.
Finally realized that on the port with the damaged cable, the system was sending out the login banner, which was promptly read back in as input from the terminal (pins 2 and 3 shorted). This generated multiple new login banners, exponentially increasing the amount of traffic until the PDP11 (which was very good at handling character I/O normally), just gave up the ghost,
Once found, rather than laying a new 25 metre cable, the rather embarrassed technician cut the damaged bit of the cable out and spliced in (using screw terminal blocks, I believe) a couple of inches of new cable,
The second story is from a factory floor, where a terminal was in the middle with the cable, the maximum length permitted by the RS232 standard, was run through the roof. Periodically, a couple of times a day, the computer it was attached to reported TTY Hog messages, and promptly shut the terminal down. I was giving remote support to a VAR, and after several days getting them running diagnostics on the port and checking the cable for damage or shorts, I asked where the cable was routed. They said that they had run it down the existing cable runs with everything else. After a few seconds thinking, I asked what else was fun down the runs. "Oh", they said. "Pretty much everything". Apparently, this included power for the electric motors that ran large industrial hoists for moving things around the factory floor. "OK", I said, "Is there any chance that the ports shutting down happened at the same time that these hoists were operating,,,,".
Turns out the motors were very dirty, and drew a lot of current. The rapid current spikes generated by the motors was being picked up as interference on the RS232 cable, which was not good quality shielded cable. It would have worked fine in an office environment, but in an industrial environment, it was not up to the task. The system tried it's best to make sense of the noise but failed, shutting down the port. Anyway, they actually relocated the terminal to a different part of the factory floor with a much shorter cable run and good quality cable, and I never heard from them again.