Oldie but a goodie.
Mid-80s, I was working for a company that built gear to dynamically allocate bandwidth between video, voice and data.
Incredibly Big Monster of a company started getting weird bit error rates on their global T1 (E1, T3 etc ...) network, which was larger than the Internet itself at the time. The network used our kit to terminate the telco supplied wire, and because telcos are perfect ::coff:: & it couldn't possibly be Monster's fault, it must be our problem, right? I was assigned to track down the problem after lower level techs couldn't figure it out.
Going thru' the data, I discovered that once the problem started occurring at any one site, it gradually became worse ... It was never bad enough to actually take down a connection, but network errors ramped up over time.
Further review showed that the same team of installers had installed the gear at the sites with the problem.
I flew out to Boca and discovered that they had installed punch-down blocks in a janitor's closet ... directly over a mop bucket full of ammonia water. A couple quick calls confirmed similar placement in other offices, world wide. Seems it was the only wall space that was unused almost universally in such spaces.
Corroded metal replaced and blocks relocated, no more bit-errors ... Rather than a "thank you", I got the task of updating the installation documentation. Naturally.