Be sure your sin will find you out. Or rather, "ye will know your sin" ( וּדְעוּ חאָתְכֶם) "which shall find you out" (for מָצָא cf. Genesis 44:16). So in effect the Septuagint: γνώσεσθε τὴν ἁμαρτίαν ὑῶν, ὅταν ὑμᾶς καταλάβῃ τὰ κακά.
When they had cause to rue their folly, then they would recognize their sin.