Contradiction with Luke 6:16
Mark 3:18 names "Thaddeus" as one of the apostles, whereas Luke 6:16 lists "Judas the brother of James" instead.
Luke 6:16: And Judas [the brother] of James, and Judas Iscariot, which also was the traitor.
Contradiction with Acts 1:13
This verse mentions "Judas the brother of James" like in Luke, not "Thaddeus."
Acts 1:13: And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James [the son] of Alphaeus, and Simon Zelotes, and Judas [the brother] of James.