a costly mistake的用法和样例：
- The general's decision to invade was a costly mistake.
- Words come so naturally and become so closely associated with what they represent that we may unconsciously regard them as synonymous with reality. That can be a costly mistake.
- It was a costly mistake.
- His costly mistake resulted in severe loss.
- Passenger ships are a costly way to travel.
- A costly ornament of precious metal or gems.