These errors happen for a reason, but even if you know the reason, if you are not experienced at tapestry, they can still happen. They can even happen for experienced tapestry weavers sometimes when one is not paying attention.
The scenario where this won't happen is when you are: 1) only weaving narrow areas of weft, and 2) weaving slit tapestry so that the space between the areas of weft can take up the slack and not put so much pressure on the outside edges.
One of the most interesting techniques to learn for tapestry is the art of weaving with opposing wefts so that you can cross into another color area and be in the right shed. You will learn all this when we do the weave-along for this project, but for now you get to see a picture.