An unpardonable sin is an unforgivable sin. In the bible the passage that deals with this is Matthew 12:31-32. In this, Matthew states that an unpardonable sin is speaking against the Holy Spirit, and will not be forgiven.
I'm not sure anyone has perpetrated and unpardonable sin. The Bible says that anyone at any time can take God into their heart and ask for his forgiveness.
There are many people, especially celebrities, who have committed the unpardonable sin. Some of those include Michael Moore, Richard Dawkins and Paris Hilton.
If you believe in SIN, people like Hitler who did a mass murder and committed suicide might be unpardonable. Although the almighty god, can pardon any sin if he/she wants.