The fact that we even want to know if there is a sin that is unforgivable should show us something about our character. We want to know a limit so we can live as close to it as possible without going over. The Bible mentions one "unforgivable sin" and that is blaspheming the holy spirit (Mark 3:29 and Luke 12:10). This has always been a little unclear for me, but as a result of the book I am currently reading, I can see the full extent of this sin.
Jon Krakauer's book, Under the Banner of Heaven highlights the crimes committed by some fundamentalist Mormons, namely that of Ron and Dan Lafferty. Ron's wife had left him and there were 3 people he had mainly to blame. He began leading his brothers in meetings and at one point received a revelation from God that 3 people needed to die to atone for evil and allow the Lord's work to flow. You can guess who those people were. However, in his revelation it wasn't meant for him to do the killing but his brother Dan who believed nothing could trump the Lord's will to which he would be 100% obedient. So, they started with the first target, their sister-in-law and their 15 month-old niece. Dan had no emotion/remorse because he truly believed that the Lord told Ron the truth. Ron, however, was internally tortured and tried to commit suicide during their first imprisonment and eventually received the death penalty. Dan is still in prison. 
In this situation, I believe Ron had a personal revelation of revenge that he presented as God's to his brother thereby blaspheming the Holy Spirit. Dan acted in obedience to God as he thought but Ron began to doubt his revelation and felt guilt beyond belief. This is a horribly disturbing story and I am thankful that the Bible tells us to check and seek confirmation for anything God tells us by seeing whether it is in line with God's character, his word and through confirmations of other Christians. Although the Mormon and Christian faiths are completely different, they still have zealous members as do Buddhists and Muslims. I am not writing this to compare faiths but to shed light on the danger of giving someone a message from God when it is not. And God's law doesn't label things "sin" because they would be too much fun for his followers but because they are harmful and take away from a full life.