You've almost certainly got either a corrupted download or burn.
Did you download using the bit torrent? If not, then that is probably the reason - either use md5sum to check the download is intact or simply download again using a bit torrent client as this will ensure the integrity of the download. But it is worthwhile having md5sum handy anyway to check - details on the web site.
Once you have established the download is ok, then you need to make a good burn (this is the most common issue) because the data is a very large monolithic tar file it is particularly susceptable to fatal errors due to a bad burn. The simple answer is burn at the slowest possible speed your DVD drive allows, and no jumping around the house while it goes!