Omitted-variable bias and why more data won't save you
You regress wages on years of education by OLS, but omit innate ability, which raises both education and wages. So ability sits in the error term and is correlated with your regressor.
Which classical assumption fails? What property is lost, in which direction is the coefficient biased, and why does collecting more data not fix it?
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.