Faults are minor negative modifiers bestowed on a player for 3 hours following being attacked with a Sabotage Action. The modifiers are usually -1% or -2% on attacks or defences for a random Action type. For example, you may get -1% on all Barter defenses.
Faults are essentially the opposite of the Virtues granted by Aid Actions.
Given the fact that the modifiers are small, completely random and short-lasting (although some Talents may slightly increase the duration above 3 hours), Sabotage is not considered to be a particularly useful attack type.