Perhaps they were not thinking. Maybe they did not realise it was a secret. Maybe their mouth started before their brain. It could be that they say the secret and only then realise what they have done. Or perhaps it was on purpose. Whatever the reason, your character has let the cat out of the proverbial bag. The secret is out there.

Feelings might be hurt, lives could be in danger, an argument might be fast approaching, or maybe a surprise birthday party just got spoiled. What sort of secret is your character going to speak today?