Artist's commentary
"さ": Even Monkeys Fall from Trees
Pronounced as "saru mo ki kara ochiru"... An expression for the fact that even the most skilled and talented can still fail at their specialty from time to time.
pixiv #30977989 »←"こ" "し"→pixiv #31016815 »
Also tagged with "#河童の川流れ", or "#Even Kappas can drown", which is another common saying that means "even experts can make mistakes", as well as a pun on Nitori's species.
T/n: Nitori's mistake as depicted here is the "二歩/nifu" illegal move, in which a pawn is dropped on the same column as another pawn.
(Source: Shogi drop rules, 二歩/nifu illegal move)