When Arthur comes back from China, he brings his mom his dead father's watch, which has the inscription D.N.F. engraved on it, which stands for "Do Not Forget." That's all well and good, but this phrase is so cryptic and hard to interpret that whatever Arthur's father might have meant is lost in translation. Which sort of raises the question – a watch with initials? Really? Couldn't old Mr. Clennam have just written a letter?
In any case, the whole Mrs. Clennam subplot rests on how she's supposed to read the phrase "Do Not Forget." Hint: she doesn't get it right. For her, a woman who's been convinced all her life that she is some sort of God's vengeance on earth, "Do Not Forget" means "don't forget all those horrible sins that you're supposed to keep on avenging till you die." And she doesn't. She never forgives Arthur's father, never loves Arthur or treats him as a son, and never tries to make amends for keeping the Dorrit family needlessly locked away in prison. For her, the fact that she is in a wheelchair makes it OK for the Dorrits to rot in the Marshalsea.
Sadly, though, what Arthur's dad meant by "Do Not Forget" was "don't forget to make some restitution for all the evil you've done, which I have gone along with." These two interpretations are at the heart of Mrs. Clennam's total misunderstanding of Christianity as Dickens saw it: instead of a religion of mercy, love, and forgiveness, she sees only punishment, vengeance, and eye-for-an-eye justice.