I did some serious shopping at the Labor Day weekend sales this year, including buying two balls of Noro King for a shawlette. My basic geometry rule for right triangle shawls knit from the center neck outward is each successive ball of yarn will create half as many rows as its predecessor. This was a basic garter stitch shawl: cast on 3 stitches provisionally, knit 3 garter ridges, pick up 3 stitches from one side, pick up the 3 cast on stitches, and knit every row (right side rows are increased a few stitches in from the each end and on either side of the center stitch, wrong side rows are knit plain).
Just to be safe, I stopped two rows early to give myself extra yarn. I also made the edging a few stitches narrower for the same reason.
I ran out of yarn with less than 6" of edging left to complete.
The only thing I could figure out might have happened is that Noro is sometimes less than precise in its windoffs (I once bought a ball of Silk Garden Sock that weighed 82 grams), and maybe ball #2 had less yarn than #1.
I returned to Yarndogs to buy another ball (they had several more when I left the sale), but they were all sold out.
At this point I contemplated ripping out my entire edging and reknitting it slightly narrower. I thought about how that might make the yarn look worse. I thought about how much I liked the way the shawl looked now.
I bought the two balls (thinking the leftover could be a scarf) and was pleasantly surprised when they arrived. The colors looked very similar to what I had, and I was able to finish the shawlette quickly.