In The Last Supper, da Vinci originally depicted a saltcellar knocked over and spilling out its contents on the table. This image has been reduced to a brown blob today, but early copies of Da Vinci's work showed it as spilled salt.
The common explanation has been that in folklore dating back as far as the Roman Empire, spilled salt is an omen of bad luck. However, scholar Jack Wasserman, in a 2003 article called "Leonardo da Vinci's Last Supper: the Case of the Overturned Salt Cellar" in Artibus et Historiae, ties the spilled salt more closely to New Testament theology. Wasserman first points to the passage in the gospel of Matthew 5:13, in which Jesus tells the disciples,
But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything... (NIV)
Wasserman then explains that early church theologians such as Origen, who Wasserman says were widely read in da Vinci's time period, explained good salt as a symbol of grace—of true, active faith in the word of God. The spilled salt would be no good and would symbolize Judas's loss of faith in Jesus. Wasserman also notes that the work depicts a moment of discord, when Jesus has just told the disciples that one of them will betray him. According to Wasserman, the spilled salt is a symbol, too, of this discord and lack of fellowship.