Last year when I took my '97 300TDi for it's MOT I mentioned that the reversing light didn't work, I had changed the bulb and it had not worked so there was a reversing light switch on the passenger seat if they needed to change it.
As it turns out the reversing light is not an MOTable item so it passed.

Roll on a year and I give my Landy it's pre MOT service. After the usual oil and filter changes I have a look at where the reversing light switch is! Would be easier to get at if I remove the centre cubby box but to do that I need help as it is bolted through the seat box. So I decided to put a meter on the reversing light bulb to make sure the switch is faulty. I slip it in reverse and as I wobble the light lens the light flashes on! A close look shows the bulb is not installed properly....duh. It's only taken me a year to replace a bulb......

Must check things properly the first time!

I always feel better when I've broadcast some of the dummy things I've done occasionally. I remember rebuilding a highly tuned Escort engine and almost started it before I remembered I hadn't put any oil in the sump.

I once rebuilt my sister in laws Chevette engine but could not get it run - just a backfire. It was only while sitting thinking about it a few days later and going through all the settings in my mind that I remembered I had set the tappits to 20thou......with my metric feeler gauges! So, that would be 0.2mm then!!

Years ago I helped a friend do an engine swap on his Vauxhall. In our enthusiasm to see if the new engine would start (it did!) we had forgotten to put the oil filter on. I remember being puzzled at what the jet of dark stuff was, decorating the engine bay. It took me a moment or two to realise that it was the oil, coming out at a healthy 60 psi. We also forgot to tighten the earth strap back onto the body shell, so the bolt welded itself into the hole with about 1/2" still to go. Happy days.

Buy a multimeter for electrical work, they cost a fiver on the bay.

Got several multimeters. That was what I was going to use to prove the switch was faulty!

Used to work for IBM as a Technical Specialist so not used to getting greasy!

Exactly the same happened to me. Noticed it wasn't working after I bought it, then took the cover off, gave the bulb a wiggle and felt as though it located itself. Hey presto, the reverse bulb worked!