One of the worst bits of DIY I ever saw was on a job where the customer swore blind that the lights had worked fine up to a few weeks prior to my visit.
I opened up the 3G switch and found every single wire in the wrong place. They'd also put up two new light fittings in the kitchen and wired them up wrong.
Nothing worked, some lights were in series with others and when the kitchen lights were on, the MCB popped out.
I politely informed her that "never in a month of Sundays would that lot EVER have worked". She stared me straight in the eye and swore blind it did.
How she had the brazen cheek I do not know.
Best one I ever had was a new extension working with another spark during my training. Did most of the work on that job. We finished up left get a phone call later on saying there was no power. I went checked the CU, RCD was tripping 16th install so the ring mains were on the RCD side. We were confused for about an hour as we knew it was all brand new cables. I suspected as it was one of my first jobs I did something wrong.
IR'ed one leg of the new ring and the results were suspect with the orginal results. Something did not add up. There was a N to E to L short.
Asked what work had happened since I left. Answer was nothing we just started to move into the new extension.
We walked around took socket fronts off. Then I noticed a clock in line with a socket under it above a work top. No cuboards just a work surface so the cables go up into the floor in a safe zone.
Take the clock off the wall. Customer still denys anything. Rip the new plaster off the wall, hole in capping, screwed right into the new cables.
Customer swears its my fault me been a new spark. Threatens my boss, so he says how can I be responsable for a clock you screwed into a cable because you did not think a cable would run that way in a wall. Customer still denys the clock was the problem and was all ways there. Tried to explain no way as we did first fix then second fix so the walls were bare.
She got a little arogant with me so I left the hole I was going to repair / replace the cables and re cert and plaster for them that night. Left them with power by removing the legs from the ring main and said I would be back when I some one decides who is paying for the damage. They were insistant we should pay as we did the install and ovb was not to good standards.
Two days they spent denying they ever put that clock up.
They eventually said they put the clock up and drilled the hole when my boss said if they refused to pay and was asking me to remove our breakers from the CU (we fitted a new CU at the same time for the house) while he was talking to them. Threat of no power soon made them cough up.
Adam