Lord's Prayer Doxology Missing

The ending of the Lord's Prayer, the doxology, is missing from older New Testament manuscripts. Most Christians will be familiar with this last verse: "For thine is the kingdom, the power and the glory, for ever and ever, Amen". But in the Codex Sinaiticus it's missing from the Gospel of Luke. Can this be just a careless mistake? Some say it's because the Codex Sinaiticus is unreliable and full of errors.