Gauge and not using the yarn the pattern calls for. Have you heard that one before?
I've started on a new dress yesterday. Measured, had the gauge (so I thought) and started knitting. Small was to tight, or was it. Normally I would have started /used small, but yesterday I decided upon the bigger size. Casted on 280 stitches (large needles) and knitted and watched biathlon. Probably more watching TV than looking at the knitting.
This morning, when looking at the first 5-8 cm, it did look big. So I measured and measured and yes it was going to be a tent and not a slim, fitted dress.
What did I do, started thinking about how to decrease in to make it a fit, and knitted 2 more cm.
Then I realized what I was doing - knitting a lot more stitches than needed (on a dress), making it to big and I was only about 8 cm up on the knitting. So I frogged and restarted, and am indeed happy with myself.