It's easy to miss. But, IMO, a bit critical.
Actually, that kind of describes so very much of the series in a nutshell: "easy to miss, but critical."
SpoilerLike how there's a chance that Harry has been infected by Nemesis/The Adversarysince, at the very least, Bianca's party way back in book 3 maybe. Possibly since his time with Justin du Morne.