Sounds like the start of a very dirty joke, no? Actually, these are the two biggest clues that Help (the book within The Help) is set in Jackson, Mississippi. The fact that Minny baked a poo pie that Hilly ate is left in Help intentionally, to keep Hilly from spreading the word that the book is about Jackson. If she admits the book is about Jackson, she admits she ate Minny's poo…and lived. (The big irony, of course, is that Hilly claims to believe that white people will come to harm if they use the same toilets and dishes as black people. If a white person can eat a black person's poo and not even get sick…we'll leave you to finish that sentence.)
By contrast, Aibileen's inclusion of the fact that the Leefolts' dining room table has an L-shaped crack in it is totally unintentional, and a sign of Aibileen's attention to detail (at least in her descriptions, though perhaps not her editing). But, the inclusion might have an unintentionally beneficial result. We could look at Aibileen getting fired as a good thing – after all, she gets the Miss Myrna job and income from Help. Moments after she's fired, she frees herself to begin planning her new writing career.
The inclusion of the L-shaped crack might also have benefits for Elizabeth. Until Hilly points out the detail of the crack, Elizabeth somehow doesn't recognize herself in Aibileen's story. Now she'll be forced to deal with Aibileen's vision of her. Perhaps this will help her with some of her problems and issues. This detail also gives her some power over Hilly – something she's never had, or never felt she had before. When Hilly clues in Elizabeth that she's featured in the book, Hilly, by implication, admits that she too is featured, as the woman who ate Minny's special chocolate pie. Seeing Hilly as fallible might help Elizabeth stop idolizing and following the woman, and learn to think for herself.