Spouses taking their conflict as a couple to the parenting level is definitely the biggest mistake made during divorce. This is often severely damaging to both sides and the children become the only influence that one can exercise over the other. In this way, the children become a means of scoring points, and, in the end, everyone loses.
What can become very expensive is if a party is convinced that they must fight over some principle or other, for example, forcing 50/50 custody at any cost or that the children shouldn't see the father during the week at all. I always try to explain to the parties that this is not a mathematical equation, and everyone involved must be able to live with the ruling in their day-to-day lives.