Continuity mistake: When Graystripe and Firestar spy on the Tigerclan camp they name the cats in the clearing. Mudfur is among them but on the next page it mentions a lack of medicine cats. Mudfur is the medicine cat. As far as I know you can't unbecome a medicine cat.
Continuity mistake: Near the end when they are about to do battle with BloodClan, Firestar meets Whitestorm's "worried blue eyes" eyes. In the first book they are "the color of sunbaked sand". Cat eyes don't change colour.
Factual error: When Firestar, Ravenpaw and Greystripe are helping Mistyfoot, Featherpaw and Stormpaw escape the Tigerclan camp, it says: "the whites of his [Greystripe] eyes bright in the half light" but cats' eyes don't have whites. Their eyes are all one colour eg: orange, yellow, green. with a black pupil in the middle.