... I'd crimped one of the plugs on upside down. Could make out which is was through the translucent plastic of the plug, so five minutes later and one scrap plug, all was good.
You've reminded me of an "incident" from a good few years ago. My employer at the time managed a campus WAN for a science park - each office/unit was pre-wired with a couple of network points, and one was cabled back to a switch port. We could remotely provision internet access just by configuring the port and allocating them an IP address or range.
Unfortunately, wherever you put these sockets, it's going to be in the wrong place for someone - and sure enough, a business moved in but needed the internet connection to their server room the other side of the unit. So one 40m cable and a few lifted floor tiles later, internet provided where it was needed. Only it "didn't work".
We ended up on a 4 way call - me, the people who at the time managed the network for us, the customer's IT guy, and their external IT support. It turned out, the guy "knew" he needed a crossover cable so cut the plug off and fitted a new one wired as a crossover. - neither our switch, nor his firewall had auto MDI (most devices will automatically switch modes as needed these days so crossover cables are rarely needed, this was some time ago).
The customer's IT guy talked him through finding another crossover cable and an inline coupler. The customer said he'd found one and "it's yellow, is that OK ?" His IT guy responded as I'd have loved to ... "I don't f***in care what colour it is as long as it's crossover".
Needless to say, with the extra crossover in line, our "it's not working" service sprang into life. I managed to refrain from "I told you so" (or something to that effect).