Contradiction with Exodus 20:16
This verse instructs not to bear false witness against your neighbor, contradicting the action of slandering mentioned in Psalms 50:20.
Exodus 20:16: Thou shalt not bear false witness against thy neighbour.
Contradiction with Ephesians 4:31
Encourages putting away slander and malice, in opposition to the act of speaking against your brother in Psalms 50:20.
Ephesians 4:31: Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice:
Contradiction with Leviticus 19:16
Commands not to go around as a talebearer among your people, contradicting the slandering of a brother mentioned in Psalms 50:20.
Leviticus 19:16: Thou shalt not go up and down [as] a talebearer among thy people: neither shalt thou stand against the blood of thy neighbour: I [am] the LORD.
Contradiction with James 4:11
Advises against speaking evil of each other, directly opposing the act described in Psalms 50:20.
James 4:11: Speak not evil one of another, brethren. He that speaketh evil of [his] brother, and judgeth his brother, speaketh evil of the law, and judgeth the law: but if thou judge the law, thou art not a doer of the law, but a judge.