(They did it) their way...
To some people data cabling is just a bunch of copper wires from point A to point B. They refuse to believe that the performance of their IT systems has anything to do with it.
One such site (I won't name them to spare them the embarrassment) did it their way. The work to have structured cabling installed by qualified installers (project managed by KITS as part of a bigger infrastructure install) using warranted cabling and tested using calibrated test equipment was "way too expensive", "our electrician can do that". So he did.
He obviously has a flair for this work (but I won't be looking for him to do anything for my company).
However whilst the customer saved money on the cabling implementation it hasn't helped them one bit. Everything works but everything does not work all the time. If the site's boiler starts up it now brings down the computer network.
Having spent a couple of hours troubleshooting I finally worked out what was occurring. The electrical feed to the pump connected to the main heating boiler runs above a suspended ceiling. To the electrician it was the ideal route for the data cabling connecting the server to the internet router and the mains cable (to him) was ideal to cable tie the data cabling to ! Thus the electrical noise on the mains cable is passed over to the data cabling running along it.
The folly of such cost cutting has been pointed out, the particular section of cabling will be re-routed and, any further issues with the infrastructure will not be dealt with unless the site owner can now prove the issue is not to do with the data cabling.