Contradiction with Psalms 22:1
While Psalms 71:8 expresses praise and fills the speaker’s mouth with God's glory, Psalms 22:1 begins with a feeling of abandonment by God.
Psalms 22:1: [To the chief Musician upon Aijeleth Shahar, A Psalm of David.] My God, my God, why hast thou forsaken me? [why art thou so] far from helping me, [and from] the words of my roaring? [Aijeleth...: or, the hind of the morning] [helping...: Heb. my salvation]
Contradiction with Lamentations 3:8
Psalms 71:8 is filled with expressions of God's praise, whereas Lamentations 3:8 speaks of a situation where even prayers to God seem blocked.
Lamentations 3:8: Also when I cry and shout, he shutteth out my prayer.
Contradiction with Job 19:7
Psalms 71:8 is about constantly praising God, whereas Job 19:7 reflects a cry for help where justice seems absent.
Job 19:7: Behold, I cry out of wrong, but I am not heard: I cry aloud, but [there is] no judgment. [wrong: or, violence]
Contradiction with Isaiah 64:6
Psalms 71:8 emphasizes continual praise of God's righteousness, but Isaiah 64:6 describes the righteousness of people as "filthy rags."
Isaiah 64:6: But we are all as an unclean [thing], and all our righteousnesses [are] as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.
Contradiction with Habakkuk 1:2
While Psalms 71:8 is about declaring God's praise all day long, Habakkuk 1:2 questions how long cries to God will go unanswered.
Habakkuk 1:2: O LORD, how long shall I cry, and thou wilt not hear! [even] cry out unto thee [of] violence, and thou wilt not save!