For those who are either unaware of or who militantly deny the danger of false witness:

http://www.newadvent.org/cathen/09210a.htm

From the Baltimore Catechism:
264. What is the eighth commandment of God?
The eighth commandment of God is: Thou shalt not bear false witness against thy neighbor.

Thou shalt not bear false witness against thy neighbor. (Exodus 20:16)

265. What are we commanded by the eighth commandment?
By the eighth commandment we are commanded to speak the truth in all things, but especially in what concerns the good name and honor of others.

Wherefore, put away lying and speak truth each one with his neighbor, because we are members of one another. (Ephesians 4:25)

266. What does the eighth commandment forbid?
The eighth commandment forbids lies, rash judgment, detraction, calumny, and the telling of secrets we are bound to keep.

Lying lips are an abomination to the Lord. (Proverbs 12:22)

267. When does a person commit the sin of rash judgment?
A person commits the sin of rash judgment when, without sufficient reason, he believes something harmful to another's character.

Before thou inquire, blame no man. (Ecclesiasticus 11:7)

268. When does a person commit the sin of detraction?
A person commits the sin of detraction when, without a good reason, he makes known the hidden faults of another.

A good name is better than great riches, and good favor is above silver and gold. (Proverbs 22:1)

269. When does a person commit the sin of calumny or slander?
A person commits the sin of calumny or slander when by lying he injures the good name of another.

Devise not a lie against thy brother, neither do the like against thy friend. (Ecclesiasticus 7:13)