Contradiction with Matthew 24:29
Matthew 24:29 describes the same event of the sun being darkened but adds detail by saying "immediately after the tribulation," whereas Mark 13:24 just states "in those days, after that tribulation," which can be seen as less specific and thus potentially contradictory in timing.
Matthew 24:29: Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
Contradiction with Joel 2:31
Joel 2:31 mentions the sun turning into darkness "before the great and terrible day of the Lord come," which seems to suggest a different timing relative to the day of the Lord compared to Mark 13:24.
Joel 2:31: The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the LORD come.
Contradiction with Revelation 6:12
Revelation 6:12 also describes a darkened sun during the sixth seal, but in a context where stars fall to earth, potentially suggesting a different sequence of events compared to Mark 13:24.
Revelation 6:12: And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;