Lena had forgotten about forgetting Kostos. That was how she knew. When your remembered to forget, you were remembering. It was when you forgot to forget that you forgot. The thing that reminded Lena about Kostos came not from any movement in her brain (which would have constituted a failure to forget) […]
It was simple. When she saw Kostos, she remembered him.
Sisterhood of the Traveling pants: Forever in Blue - Ann Brashares