He was concerned 'lest' anyone think that he was guilty. = ('chiefly Brit') He was concerned 'lest' [='in case'] anyone should think he was guilty. [=he was concerned that people would think he was guilty; he did not want anyone to think he was guilty]
She's a talented singer and, 'lest we forget', a fine musician as well. [=we should not forget that she is also a fine musician]
And 'lest' you think I'm joking, let me assure you that everything I've said is true.
(..) ...they conquered. But what am I saying? You guys are pros. The best. I'm sure you can make it out of the casino. Of course, lest we forget... ...once you're out, you're still in the middle of the fucking desert! You're right. He's right. Our eyes were bigger than our stomachs. (..)2001 Ocean's Eleven