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.
I am short at least an entire skein's worth of yarn.
So much for calculating the difference. Yes, I know. I suck at math; but I didn't think I was that poor at it. What's worse is, I have another shawl with the same yarn (different color, of course) waiting in the wings. Which, of course, also will be one skein short. And I can't even get a head start on knitting that, because I only have one size 15's, and they are currently being used by the shawl already on the needles.
So, what's a girl to do? Why, start another project, of course! I know I have a few projects I could be working on, but I wanted something smallish and fairly quick to knit. Again, I was mistaken. Size 1 DPN's make for tiny porcupine knitting, which is decidedly not fast. Still, the project is small, so it shouldn't take me too long - I hope.
The idea was to make several book scarves, but with how fiddly this is turning out to be (again, I say: tiny porcupine knitting), I'm not sure I'll be able to stomach that. Then again, they're gifts, and it is still over 300 days 'til Christmas, so maybe if I spread it out over the year, it won't be as bad. Though I may see if I can get circulars in the right size, instead. Less painful ;-)
Also, I just have to say I love my hubby. One of the skeins of DMC yarn I was trying to get split and wound up was all tangled to the point where I couldn't do anything with it anymore. He took it, and in the end I only lost about three inches worth (as opposed to the entire thing). Woot!