Symbolism, Imagery, Allegory
In Lena and Harold's guest bedroom is a is a poorly designed marble end table, topped with a black vase. This monstrosity is all thanks to Harold: he designed the useless thing.
The table starts to tip over if you barely touch it—it’s destined to fall. In fact, it does fall, and the vase breaks. Lena responds to the accident by saying that she knew this would happen Her mother (quite sensibly) asks why she didn’t do anything to prevent it.
The not-so-hidden-symbolism here: the table and vase are metaphors for Lena's marriage. The marriage, like the vase, is incredibly fragile. And, like the end table, it's rickety and unstable. (Bonus symbolism points: it's mainly Harold's fault that the marriage, like the table, is unsteady.)
All of this is underlined by the fact that this table-vase combo goes to pieces at the very point that Lena starts questioning the stability of her marriage. Lena can see that her marriage is heading to disaster. The question is: will she do anything to prevent it?