This is perhaps the most blatant metaphor ever. There is a poorly designed (by Harold) marble end table in Lena’s guest bedroom with a black vase on it. 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 says she knew it was going to happen. Her mother asks why she didn’t do anything to prevent it.
So this table falling and vase is a metaphor for…Lena's marriage. The marriage is the vase. It’s fragile, but also sitting on a really rickety foundation – the table designed by Harold (a.k.a. the foundation of their marriage, which is the idea of equality and balance sheets, also designed by Harold). So when the foundation gets the slightest nudge, the marriage shatters. Notice that the table falls and the vase breaks just as Lena starts questioning the foundation of their marriage: "We need to think of what our marriage is really based on…not this balance sheet, who owes who what." So that’s the nudge to the foundation. Lena can see that their marriage is heading to disaster. The question is: will she do anything to prevent it?