I often notice distortions of this kind in my drawings. The problem is that I often fail to notice these; there's a trick (just in case you haven't heard of it yet) to check your drawing regularly from a different point of view: temporarily flipping horizontally (if digital), in a mirror, from the other side of paper*, or just rotating your sheet. This lets you to see errors that your eye became used to.
I suppose distortions like above may be because of incorrect posture while drawing: I lean to my left, so I see the drawing at some angle. (here the picture is already flipped horizontally, so it's as if I leaned right; the drawing is elongated along the axis that goes from closest ear to nose of the fox)
Bad thing when I forget/hurry to check it earlier, so I notice the error after I've already inked and scanned. Then, I try to fix it by using Shear transformation on fragments of the image. Seems to actually look less clumsy... Haha, cheating that results in neither good art or learning. But well... Did it have any sense in the first place anyway?

* That must be funny looking. Like, I'm showing my drawing to a lamp to see what it thinks of it.