Contradiction with Proverbs 10:23
While Proverbs 15:21 speaks negatively about folly bringing joy to those without wisdom, Proverbs 10:23 suggests that the behavior of engaging in mischief is a distinguishing feature of the foolish.
Proverbs 10:23: [It is] as sport to a fool to do mischief: but a man of understanding hath wisdom.
Contradiction with Ecclesiastes 7:4
Proverbs 15:21 notes the joy associated with folly, yet Ecclesiastes 7:4 states that the heart of the wise is in the house of mourning, and associates being lighthearted with being foolish.
Ecclesiastes 7:4: The heart of the wise [is] in the house of mourning; but the heart of fools [is] in the house of mirth.