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).