Contradiction with Mark 16:14
Mark describes Jesus appearing to the eleven apostles as they sat at a meal, not over a period of 40 days.
Mark 16:14: Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen. [at meat: or, together]
Contradiction with Luke 24:36-43
Luke notes that Jesus appeared to the disciples in Jerusalem and ate a meal with them, implying a single event, not over 40 days.
Luke 24:36-43: And as they thus spake, Jesus himself stood in the midst of them, and saith unto them, Peace [be] unto you.
Contradiction with John 20:19-29
John recounts Jesus appearing twice to the disciples, both on a Sunday, rather than mentioning a period of 40 days.
John 20:19-29: Then the same day at evening, being the first [day] of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace [be] unto you.
Contradiction with 1 Corinthians 15:5-8
Paul mentions appearances to Cephas, the twelve, over 500 others, James, and all the apostles, but doesn't detail the extended 40-day timeline.
1 Corinthians 15:5-8: And that he was seen of Cephas, then of the twelve: