Contradiction with Psalm 115:17
This verse states that the dead do not praise the Lord, which contradicts the implication that if the psalmist goes down to the pit, he will no longer be able to praise God in Psalm 30:9.
Psalm 115:17: The dead praise not the LORD, neither any that go down into silence.
Contradiction with Ecclesiastes 9:5
This verse suggests that the dead know nothing and have no more reward, contradicting the idea in Psalm 30:9 that the dust can declare God's truth.
Ecclesiastes 9:5: For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.
Contradiction with Isaiah 38:18
This verse claims that Sheol cannot praise God, which contradicts the suggestion in Psalm 30:9 that going to the pit could end the ability to praise or speak of God's truth.
Isaiah 38:18: For the grave cannot praise thee, death can [not] celebrate thee: they that go down into the pit cannot hope for thy truth.