Here are some examples:

1.好巧啊 hǎo qiǎo a

2.真巧 zhēn qiǎo

3.太巧了吧 tài qiǎo le ba

4.怎麼那麼巧 zěn me nà me qiǎo

5.這麼巧 zhè me qiǎo

We might say "What a coincidence!" in the following situations:

1. **Meeting an acquaintance**: Running into someone you know in an unexpected place, such as in a different city or at a place you don’t often visit, like meeting a friend or colleague.

2. **Thinking of the same thing simultaneously**: Two or more people thinking or saying the same thing at the same time, for example, mentioning the same movie or restaurant during a conversation.

3. **Experiencing similar things**: Different people experiencing similar things in different places, such as two people traveling to the same destination at different times or buying the same clothing item.

4. **Coincidence of random events**: Similar events happening in completely random situations, such as different people winning the same prize in a lottery at different times, or making the same decision on the same day.

5. **Unplanned synchronization**: Two or more people making the same decision without prior discussion, like going to the same restaurant on the same day or taking leave to travel at the same time.

6. **Timing coincidences**: Something happening at a particularly coincidental moment, such as calling each other at the exact same time or thinking of someone just as they message you.

These situations can all make people say "What a coincidence!" expressing surprise and amazement at the occurrence of such coincidences.

But please be careful with your tone, it's best to say it in a friendly and happy way, so the person who hears it will also feel happy.