I got an old and rubbish bike off Freecycle recently, cleaned it up, did some work to the wheels, replaced the rear brake cable and adjusted the brakes, adjusted the saddle, refurbished the pannier rack, and cleaned and oiled the chain. Since then I have been using it for little shopping trips and have also fitted a child seat to the back and have been really enjoying going for little rides with my daughter on the back.
I was noticing quite a lot of noise from the chain, so I checked the derailleurs, and found that the rear was screwed into the dropout slightly wonky, so that the cage below is sticking out at an angle away from the frame.
I am pretty sure I did this when I was cleaning the bike - I had unscrewed the whole derailleur in order to give it a good brushing (I now realise this was almost certainly unnecessary) and I must have stuffed up the thread when refitting it.
Today's top tip: do not do this.
I bought a 10mm thread tap off ebay and have just been trying to retap the hole, which was fun. I thought I'd done it OK, but the derailleur screw just won't tighten - I think the thread is gone for good. I am wondering if it worth trying to use some horrendous glue to try to bind the screw in there permanently?
Anyway, I stuffed up and I'm not really expecting any solutions (though I would welcome them). I'm posting this here as a warning. Sympathy would be nice, but derision more appropriate. At least the bike didn't really cost anything.
I expect someone will be along to suggest turning it into a fixie