I should be glad that this doesn't happen to me often.
I bought some yarn - a greeny/turquoise aran weight - safe in the knowledge that there would be enough to make a sweater for me.
I started to make a cardigan, but didn't like it when I'd finished the yoke and frogged it. I left it in a bag to mature while I waited for inspiration. This eventually came in the form of Anhinga.
I love Nora Gaughan's designs.
Anyhow, I cast on and knitted the back and left it to mature again. I think it was the colour that wasn't really doing it for me. This holiday that guilt got to me so I picked it up and knitted the front. And ran out of yarn! I went to the yarn shop to buy some more for the sleeves and neck but the colour isn't the same. The dyelots match, but the new stock is definitely more green!
I admit defeat.
I'm frogging it and giving the yarn away.
(Anyway. I bought 1600 yards of purple aran to restart the original cardi.)