Another poster has reminded me of mine.
My worst was a long black cardigan I was making for a sister in the US, since I knew she wanted something of the sort.
Having knitted all the major bits, I then went nearly mad/blind, trying to 'pick up and knit' the right number of umpteen stitches all round the front and neck edges, for the ribbed edging.
After trying for ages - it was so hard to see black anyway, even with a good light - I finally gave up.
Never again will I embark on anything with more than a very small pick up and knit section.
Luckily the yarn was not too expensive! Had to be tumble-dryable for sister (in the US) who tumble dries absolutely everything.
What's your worst/most guilty UFO?