Contradiction with John 6:54
Mark 14:25 speaks of not drinking of the fruit of the vine until the kingdom of God, while John emphasizes the necessity of partaking in his blood for eternal life.
John 6:54: Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
Contradiction with Matthew 16:28
Mark 14:25 suggests a future coming, yet Matthew 16:28 suggests some will not die before seeing the Kingdom, implying it's already present.
Matthew 16:28: Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
Contradiction with Luke 23:43
Mark 14:25 implies a future event in God's kingdom, but Luke 23:43 indicates immediate presence in Paradise with Jesus.
Luke 23:43: And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.