False witness and perjury. When it is made publicly, a statement contrary to the truth takes on a particular gravity. In court it becomes false witness. When it is under oath, it is perjury. Acts such as these contribute to condemnation of the innocent, exoneration of the guilty, or the increased punishment of the accused. They gravely compromise the exercise of justice and the fairness of judicial decisions.
Source: Catechism of the Catholic Church (Vatican.va)