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Once bitten, twice shy 

means that when someone has had a bad experience, they are more cautious about similar situations in the future.

Examples:

1- After buying a faulty car from a second-hand dealer, Matt only buys brand-new cars now. Once bitten, twice shy.

2- The company partnered with an unreliable supplier last year. Now, they run thorough background checks on new vendors. Once bitten, twice shy.