Xmas Lights Tester
So, you accidentally stick a regular clear bulb in your Christmas lights (fairy lights [US] ?) instead of a fuse bulb. Next time there's a bit of a surge, there's no discernable weak point in your circuit and it takes out more than one bulb. Where do you start debugging that?
Sure, you could sit with a multimeter and probe all the bulbs to find the duff ones. Have you ever tried to attach multimeter probes to a fairy light bulb?!?! It's a nightmare. Specially when you only have 8 fingers and two thumbs.
My bulbs have these little skimpy connectors and I think they're enamelled, so you'd be really lucky to get a closed circuit even with a good bulb.
Knickers to that. Chop one of the bulb holders out of the set and attach it onto a pair of 4mm banana plugs. Test all 40 bulbs in about 2 minutes. Patch the bulbholder back in. As it happens, 11 of them were blown (not bad!) and I only had 5 spares. So the whole set went into file #13 (the garbage).