In DN 27, the Buddha said that the Brahmins make bad karma for speaking falsely.
“Actually, Vāseṭṭha, the brahmins are forgetting their tradition when they say this to you. For brahmin women are seen menstruating, being pregnant, giving birth, and breast-feeding. Yet even though they’re born from a brahmin womb they say: Only brahmins are the first caste; other castes are inferior. Only brahmins are the light caste; other castes are dark. Only brahmins are purified, not others. Only brahmins are Brahmā’s rightful sons, born of his mouth, born of Brahmā, created by Brahmā, heirs of Brahmā.’ They misrepresent the brahmins, speak falsely, and make much bad karma.
So my question is do you get bad karma if you say something you thought was true but turned out to be false?