While Pietro's most notable success came at Milan, an unusual incident occurred while playing for Genoa in 1937. During a Fiorentina-Genoa match won 2–1 by Genoa in Florence the linesman called a nonexistent offsides on Arcari; the referee Caironi, without hesitation sent his colleague to the locker room, who was quickly replaced. That season, he won the Coppa Italia, as Genoa defeated Roma 1–0 in the final, played in Florence, on 6 June 1937; en route to the final, he eliminated his former club Milan in the semi-finals of the competition.