In a previous job, we got a call from one of our customers one day asking if it would work to plug a lead from a generator into a socket - because they'd got a notice about a planned outage. In the past I'd been up there with my small portable genny and run an extension lead up to the server room to keep the key equipment running, but this time they'd been given another option ...
I carefully explained that such a cable is called a widowmaker for a very good reason, and in any case they had a 3 phase supply and the socket would only power 1 phase. I think I had them with "widow maker" and "very dangerous" so the next bit was an easy sell - get your electrician to fit a power inlet and a changeover switch, which they arranged at very short notice.
Anyhow, I can't say too much as it would instantly identify the businesses concerned, our client's neighbour has a rather handy trailed diesel genny (around 80kVA IIRC) that they used to power some machinery if there wasn't a suitable site supply available. And the neighbour had offered to let them have some juice from it while they would be running their own office. And it turned out that said neighbour was in the habit of using this genny with a widowmaker lead (2off 32A 5 pin red BS4343 plugs) for their own premises.
On the day of the outage we got a call - the power inlet idea hadn't worked. So I set off, picking up my genny from home on the way. By the time I got there they'd sorted it - a combination of the inlet not having been wired correctly (no, no idea of details, this was just second hand via a non-technical person) and I noticed a somewhat bodged together cable in use (safe, but two cables into the BS4343 plug and socket).
I ended up chatting with one of the guys from next door. It turns out that their equipment only needs a 3 wire supply (no neutral), so all their cables only have 4 cores (3P+E) even though they had 5 pin plugs & sockets. So that explained why they'd had to bodge two cables into the plug & socket to get our client working - lucky not to blow any kit up I guess. I think I probably made some comment that this was dangerous, but his attitude was that only they'd use the cables for their own equipment so there was no problem. At the time they were still using a widowmaker in their own premises - and I definitely made a thing about the dangers in that (I think I saw them with an inlet fitted some time later)
I was really tempted to raise a RIDDOR about both the windowmaker and the 4 core cables - but at the time it would have caused big problems for us and our client, and I already had something of a reputation for putting noses out of joint.