Choosing a candidate who cannot adapt to the local culture of the community and church. Some ministers are flexible, others are not. An inner city pastor might not do well in suburbia, nor a suburban pastor in a rural area. Concentrate on people from a similar background or who have gifts in cross-cultural adaptation. Failing to evaluate the church's true condition.
All churches have problems.
Be honest about your church's condition. Choosing a candidate because he is either exactly like or exactly opposite the former pastor. Evaluating the candidate's sermons upon ones he has chosen, not the committee. Always arrive unannounced to observe sermons. Having too large a committee.