When you run out of yarn before the project is complete. It really, really does. I am using a new (to me) yarn for the shawl I'm working on. One that I've knit a hundred times (well, not quite that many, but I've made it a lot). I took into account that I would need a little more. Even went so far as to compare it to what I normally used and made an attempt at calculating the difference.
I went up a needle size, and reduced the amount of stitches in order to compensate for the difference in the type of yarn and how it was knitting up. Technically, this should have meant I was using less yarn than I'd anticipated.