Contradiction with Psalm 73:3
Proverbs 21:12 indicates the righteous observe and cause the wicked to fall, but Psalm 73:3 shows the wicked prospering, which contradicts the expected downfall.
Psalm 73:3: For I was envious at the foolish, [when] I saw the prosperity of the wicked.
Contradiction with Ecclesiastes 8:14
While Proverbs 21:12 suggests the righteous observe justice upon the wicked, Ecclesiastes 8:14 points out the paradox of the wicked being treated righteously and the righteous suffering as the wicked.
Ecclesiastes 8:14: There is a vanity which is done upon the earth; that there be just [men], unto whom it happeneth according to the work of the wicked; again, there be wicked [men], to whom it happeneth according to the work of the righteous: I said that this also [is] vanity.
Contradiction with Job 21:7
Proverbs 21:12 suggests the righteous witness the fall of the wicked, yet Job 21:7 questions why the wicked often live, grow old, and become mighty.
Job 21:7: Wherefore do the wicked live, become old, yea, are mighty in power?
Contradiction with Jeremiah 12:1
Proverbs 21:12 implies the downfall of the wicked, yet Jeremiah 12:1 questions why the wicked prosper in their ways.
Jeremiah 12:1: Righteous [art] thou, O LORD, when I plead with thee: yet let me talk with thee of [thy] judgments: Wherefore doth the way of the wicked prosper? [wherefore] are all they happy that deal very treacherously? [talk...: or, reason the case with thee]
Contradiction with Psalm 94:3
Proverbs 21:12 shows the supposed fall of the wicked witnessed by the righteous, whereas Psalm 94:3 asks how long the wicked will triumph, suggesting their prevailing.
Psalm 94:3: LORD, how long shall the wicked, how long shall the wicked triumph?