Dr Martyn Lloyd-Jones observes:

There is nothing more irritating to the one who is undergoing discipline than a feeling that the person who is administering it is capricious and uncertain.

There is nothing more annoying to a child than the kind of parent whose moods and actions you can never predict, who is changeable, whose condition is always uncertain.

There is no worse type of parent than he who one day, in a kindly mood, is indulgent and allows the child to do almost anything it likes, but who the next day flares up in a rage if the child does scarcely anything at all …

Such a parent … fails to exercise a true and helpful discipline, and the position of the child becomes impossible.

Roger Ellsworth, What the Bible Teaches about Christian Parenting (Darlington, England: Evangelical Press, 2007), 74.