When there's something wrong with the underlying drawing you're painting to, and you just can't see what it is, it's time to make some relative measurements. Don't worry, there's no math involved. Instead, just get a scrap piece of paper and hold it up to your source, marking distances between a few 'landmarks' and looking for two features that are the same distance apart on the photo or whatever you're using to paint from. Then compare them in your painting and see if they're still the same distance apart. Sometimes it's hard to find two features the same distance apart, so you might have to select something that's, say, twice as long as something else. You may have to use several different features in different parts of the painting to find all the problems.

I illustrate the concept with pictures from Project 3, wherein I discovered an egregious drawing error and yet was able to fix it and 'save' the painting.

Here in the original photo, I discover that the distance between the farthest finger's first knuckle and the hand-end of the finger is the same as between the middle of the second knuckle of the pinkie and the heel of the hand. (The transparent white strips are the same size.) I also indicate the angle of the pinkie with (appropriately) a pink strip.	Here's what it looks like on my work in progress, when I first notice that Things Are Not Right. You'll see that the angle of the pinkie is all wrong, and I've made the heel of the hand a LOT longer than a normal hand would be.
	In the final painting, you can see I've managed to recover from the original drawing error - the distances are now correct and the angle is appropriate. See Project 3 for the complete start-to-finish demonstration of this painting.