I don't know

This is my standard answer to many thorny questions in theology.
But, today I learnt that to Jesus sometimes this is not an acceptable answer!
Luke 20:1-8 records an incident where the Pharisees questioned Jesus authority. He responds by asking them,

⁴was the baptism of John from heaven or from man?" ⁵And they discussed it with one another, saying, "If we say, 'From heaven,' he will say, 'Why did you not believe him?' ⁶But if we say, 'From man,' all the people will stone us to death, for they are convinced that John was a prophet." ⁷So they answered that they did not know where it came from.

So if we say "I don't know" for political reasons, i.e., to just try and keep everyone happy, it is a problem. However, if we are genuinely not sure of the answer I think it is appropriate and consistent with the humility required by passages such as Job 38. Furthermore, we also see how the above encounter occurred because Jesus was responding the hubris of the Pharisees' own questions.