To accidentally reveal a secret.
give the game away
To mistakenly reveal a secret.
give the lie to
To reveal or prove (something) to be false; to refute.
show one's cards
To reveal something known to oneself but previously concealed from oth …
bring to light
To expose, disclose or reveal something that was hidden or unknown.
spill the beans
To reveal a secret; to disclose information.
take to the grave
To never reveal (a secret).