Contradiction with Matthew 26:25
In Matthew 26:25, Judas refers to himself, asking Jesus, "Master, is it I?" whereas in John 13:26, Jesus openly identifies Judas without a personal inquiry from Judas.
Matthew 26:25: Then Judas, which betrayed him, answered and said, Master, is it I? He said unto him, Thou hast said.
Contradiction with Mark 14:20-21
In Mark 14:20-21, Jesus indicates only that the one who dips with Him will betray Him, without providing an explicit identification as in John 13:26.
Mark 14:20-21: And he answered and said unto them, [It is] one of the twelve, that dippeth with me in the dish.
Contradiction with Luke 22:21-23
Luke 22:21-23 notes the presence of the betrayer's hand with Jesus, but does not specify the individual openly as in John 13:26.
Luke 22:21-23: But, behold, the hand of him that betrayeth me [is] with me on the table.