Contradiction with Matthew 19:26
This verse states "With men this is impossible; but with God all things are possible," which contradicts the implication in Zechariah 8:6 that even God considers the task difficult.
Matthew 19:26: But Jesus beheld [them], and said unto them, With men this is impossible; but with God all things are possible.
Contradiction with Genesis 18:14
This verse asks, "Is any thing too hard for the LORD?" indicating that nothing is difficult for God, whereas Zechariah 8:6 suggests otherwise.
Genesis 18:14: Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.
Contradiction with Jeremiah 32:17
Here it is stated that "there is nothing too hard for thee," asserting God's limitless power, contrary to the suggestion of difficulty in Zechariah 8:6.
Jeremiah 32:17: Ah Lord GOD! behold, thou hast made the heaven and the earth by thy great power and stretched out arm, [and] there is nothing too hard for thee: [too...: or, hid from thee]
Contradiction with Luke 1:37
This verse says, "For with God nothing shall be impossible," which contradicts the notion of anything being difficult for God as suggested in Zechariah 8:6.
Luke 1:37: For with God nothing shall be impossible.
Contradiction with Job 42:2
"I know that thou canst do every thing," affirms God's ability to do all things, whereas Zechariah 8:6 implies something might be hard for God.
Job 42:2: I know that thou canst do every [thing], and [that] no thought can be withholden from thee. [no thought can be...: or, no thought of thine can be hindered]