What Secrets Are OK to Keep From a Significant Other?

I think it’s safe to say many, many, people would subscribe to a belief that there should be no secrets in a romantic relationship (and certainly not in a marriage) but…

Is this true? What secrets, if any, are safe to keep? Are all secrets necessarily bad? Can simply having learned from the mistakes of a past decision be enough or do you owe it to share that mistake with everybody you ever commit to the rest of your life?

Could you be in a relationship with somebody who you KNEW had secrets and who said they would not disclose them to you? Do YOU keep any secrets from the one you love?

Updates
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Things my wife and I keep from each other - in a mutually agreed and understood way: How much she spends on shoes and Amazon…. how much I spend on suits, watches, or guns.
What Secrets Are OK to Keep From a Significant Other?
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