We have been adopted by a female stray kitten. We took her to the vet to see if she was chipped (very unlikely, despite the fact its now the law here) and as she isn't we are keeping her. Hamlets here tend to be either cat places or dog places, so you either see a bunch of strays/owned cats of the very small variety or you see dogs lying in the road! It turns out our kitten is at least a year old, the vet can't tell if she has had kittens but did an ultrasound to check she wasn't pregnant, which was quite interesting, never thought I'd be looking at a cat's insides! We have to wait till she has had a season before we can get her spayed. Last night was a first as Little Cat (she weighed 2.7kg at first, about a month ago she went up to 3kg, and she is back for a booster jab today, so I expect she is a bit fatter again) stayed in with us for most of the night - she is still quite independent and prefers to go off somewhere to sleep. She must have been handled well somewhere along the line, because she is very keen on lap sitting, on her own terms, and lying about close to humans, although she also spends a lot of time alone and is still quick to scratch. Our other cat (male) is both fascinated (tries to lick her nose) and hostile. She is just hostile!
I haven't had any breeding forays except poultry, and some of those didn't work out, but I consider my greatest animal failure to be looking after an exceptionally lively 3month old German Shorthaired Pointer for a week. It was a disaster, one from which we have been put off keeping a dog forever, and worse than that, there is a permanent estrangement from the owners.
Rereading HG's original post, I have had plenty of failures raising plants, but because I live in the sun and some plants just grow without intervention, I have some very interesting cross-bred calendula (pot marigolds), californian poppies and cosmos in colours and shapes they didn't start out as. Although the plants are result of pure chance and nothing to do with my skills, I hope I fulfill a role as the audience, I do notice the oddities, they give me a lot of pleasure, and I try to save seed and perpetuate them, although this doesn't seem to work very well.
