I thought that..., I didn't know that...
The expression -(으)ㄴ/는/(으)ㄹ 줄 몰랐다 is used to end a clause stating something the speaker was not aware of or did not anticipate. -(으)ㄴ/는/(으)ㄹ 줄 알았다 on the other hand, is used to end a clause stating what the speaker had assumed to be the case; therefore, it expresses the speaker's realization that his or her assumption was incorrect.