Child benefit.

There's an inversely proportional relationship between cost and fairness.  The fairest systems are the most expensive - the cheapest systems are the most inequitable.  So it depends on what your priorities are.  Clearly the coalition's priority is to save money, so they've gone for the cheap and simple approach of saying that higher rate tax payers will lose their child benefit, even though this will produce perceived unfairness in some circumstances.  The alternative (assuming you accept the principle that child benefit should no longer be universal) would have been means testing, which, while producing a fairer result, would have been horrendously expensive.