Interfering with God's Plans.

Jacob and Esau grew to be men. They were of opposite personality and character. 

"Once when Jacob was cooking some stew, Esau came in from the open country, famished. Esau said to Jacob, "I'm starved! Give me some of that red stew!" 

 "All right," Jacob replied, "but trade me your rights as the firstborn son."

"Look, I am about to die," Esau said. "What good is the birthright to me?"

So, he sold his rights to Jacob as firstborn.
Genesis 25:29-34

Matthew Henry's Concise Commentary

25:29-34 We have here the bargain made between Jacob and Esau about the right, which was Esau's by birth, but Jacob's by promise. It was for a spiritual privilege; and we see Jacob's desire of the birth-right, but he sought to obtain it by crooked courses, not like his character as a plain man. He was right, that he coveted earnestly the best gifts; he was wrong, that he took advantage of his brother's need.

http://bible.cc/genesis/25-32.htm

The Bible says, "So Esau despised his birthright."

Their father Isaac ( probably not knowing about his son's transaction) felt he was going to die soon, so he called Esau to him and said, "Prepare me the kind of tasty food I like and bring it to me to eat, so that I may give you my blessing before I die."

His mother Rebekah, overheard the conversation and told Jacob she would cook the food and that Jacob should go in to Isaac pretending he was Esau so that he would receive the blessing instead of his brother. And although Jacob argued with her, he finally gave in and lied to his father to take the blessings from Esau.

"Isaac trembled exceedingly - Those that follow the choice of their own affections rather than the dictates of the Divine will, involve themselves in such perplexities as these. But he soon recovers himself, and ratifies the blessing he had given to Jacob, I have blessed him, and he shall be blessed - He might have recalled it, but now at last he is sensible he was in an error when he designed it for Esau. Either recollecting the Divine oracle, or having found himself more than ordinarily filled with the Holy Ghost when he gave the blessing to Jacob, he perceived that God did as it were say Amen to it."

http://bible.cc/genesis/27-33.htm

"Esau held a grudge against Jacob because of the blessing his father had given him. He said to himself, "The days of mourning for my father are near; then I will kill my brother Jacob." Genesis 27:41

God had told Rebekah that Esau would serve Jacob. Isaac chose to ignore this and wanted Esau to be the head of the family. Rebekah resorted to lies to make Jacob the head. Jacob had thought he could take advantage of Esau's hunger and buy the blessing. 

None of them trusted in God about this.  None of them waited for God's will to be done in His own way. Rebekah paid for this by never seeing her beloved son Jacob again. Jacob paid by having to run away from the murderous anger of Esau. It is never a good idea to try to make good things happen in an unrighteous way.

Don't Show Favoritism.

"The boys grew up, and Esau became a skillful hunter, a man of the open country, while Jacob was a quiet man, staying among the tents. 

Isaac, who had a taste for wild game, loved Esau, but Rebekah loved Jacob." Genesis 25:28

Isaac and Rebekah were doing wrong in preferring one child over another. This can lead to anger in children that carries over to adulthood.  God's love should flow through us to our children in equal measure - if we have trouble with one of our children we should pray that God will give us patience and deep love for that child.

Clarke's Commentary on the Bible

Isaac loved Esau - but Rebekah loved Jacob - This is an early proof of unwarrantable parental attachment to one child in preference to another. Isaac loved Esau, and Rebekah loved Jacob; and in consequence of this the interests of the family were divided, and the house set in opposition to itself. The fruits of this unreasonable and foolish attachment were afterwards seen in a long catalogue of both natural and moral evils among the descendants of both families.

God Answers Our Questions.

Isaac married Rebekah but she couldn't get pregnant so they prayed and God gave her not one, but two babies in her pregnancy. But Rebekah had trouble as the babies grew:

"The babies jostled each other within her, and she said, "Why is this happening to me?" So she went to inquire of the LORD."

Keil and Delitzsch Biblical Commentary on the Old Testament

When Rebekah conceived, the children struggled together in her womb. In this she saw an evil omen, that the pregnancy so long desired and entreated of Jehovah would bring misfortune, and that the fruit of her womb might not after all secure the blessing of the divine promise; so that in intense excitement she cried out, "If it be so, wherefore am I?" i.e., why am I alive? cf. Genesis 27:46. But she sought counsel from God: she went to inquire of Jehovah. Where and how she looked for a divine revelation in the matter, is not recorded, and therefore cannot be determined with certainty. Some suppose that it was by prayer and sacrifice at a place dedicated to Jehovah. Others imagine that she applied to a prophet - to Abraham, Melchizedek, or Shem (Luther); a frequent custom in Israel afterwards (1 Samuel 9:9), but not probable in the patriarchal age.

"The LORD said to her, "Two nations are in your womb, and two peoples from within you will be separated; one people will be stronger than the other, and the older will serve the younger."

Genesis 25:23

Thus she gave birth to twins: Jacob and Esau. This was twenty years after she had married. It was a long wait, but God did give the couple the children they wanted. Not only that, when Rebekah was upset and confused, she went to God and asked Him what was wrong. He answered her - how we are not told. But I know in my own life, when there is something I don't understand and ask God to explain things to me, he always does; one way or another. :)

I'm so glad we have such a wonderful God to go to when we are confused.