Marcos 8:12

And he sighed deeply in his spirit, and saith, Why doth this generation seek a sign? verily I say unto you. There shall no sign be given unto this generation.

There shall no sign be given ... This means "no sign like they wanted" would be given and does not conflict in any way with the exceptions cited in Matthew and Luke, nor should it be supposed that the other two evangelists were reporting exactly the same incident as here. Such a demand by the Pharisees was probably made over and over. Matthew made an exception in that "the sign of the prophet Jonah" would indeed be given that generation, as did also Luke (Matthew 12:38f; Luke 11:29f). Mark's inclusion of this demand in a different context, strongly suggests that this Pharisaical demand was repeated. It would not have been right for Jesus to have yielded to such a demand, and therefore he refused. It would also have been utterly futile to have yielded. As Luke recorded, Jesus said, "If they hear not Moses and the prophets, neither will they be persuaded, if one rise from the dead" (Luke 16:31).