Contradiction with Proverbs 12:19
Proverbs 12:19 suggests that truthful lips endure forever, while Proverbs 21:28 implies that a false witness will eventually be destroyed, which may seem contradictory if one considers the endurance of any false witness.
Proverbs 12:19: The lip of truth shall be established for ever: but a lying tongue [is] but for a moment.
Contradiction with Proverbs 19:5
Proverbs 19:5 states that a false witness will not go unpunished, which aligns with the notion of consequence, but might be seen as contrasting the finality observed in Proverbs 21:28.
Proverbs 19:5: A false witness shall not be unpunished, and [he that] speaketh lies shall not escape. [unpunished: Heb. held innocent]
Contradiction with Isaiah 5:23
Isaiah 5:23 speaks of justifying the wicked for a reward, potentially allowing the false witness a place, which contrasts with Proverbs 21:28's context of ultimate destruction for falsehood.
Isaiah 5:23: Which justify the wicked for reward, and take away the righteousness of the righteous from him!