Used to suggest someone rethink their opinion or plan, often because new information has come to light. You're sure this is the fastest route? You might want to think again—there's been an accident on the highway.
Pro Tip 2/3
Gentle Disagreement
A polite way to express that you disagree with someone, without being confrontational. You say it's a waste of time? I think you should think again; it could be beneficial.
Pro Tip 3/3
Indirect Refusal
Implies a 'no' without saying it directly, making it a softer way to decline. If you're expecting me to lend you money again, think again.