One of the things I've found in my own drawings, as well as countless other artists, is a subconscious attempt to straighten up a figure. If the head is tilted, we unconsciously try to straighten it up, usually with laughable results. I did it to my poor wife last week - I was doing a portrait, but her head was slightly tilted. What I wound up with was a good likeness of her tilted eyes, a good likeness of her tilted nose, and a good likeness of her tilted mouth ... however, they were arranged in a vertical line rather than the tilted line of her head. This made her look as if the lower part of her head had been pulled about two inches to one side. Ooops! Not a good thing to do if you're hoping for a little action later that night! I scrubbed the whole portrait out and then did the sketch of her sleeping that was posted here last week.
I saw a similar thing start to happen with this week's sketch. As you can see, the model was leaning on one arm. I had painted her shoulders, waist, and hips tilted, but then unconsciously arranged them in a vertical line. A paper towel and some mineral spirits wiped it all out again, and the next try came out much better. The moral of this story: if you know how you tend to screw things up, you're better equipped to watch for them.