Thanks @BrandonNestor for the first fact, @ShaeleighMarie for the second and 1warriorcatfan for the last two!
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-On page 214 in The Last Hope, they mistakenly call Mosskit a he.

-In Outcast on page 36 instead of using Foxpaw they say Foxkit when he had already been made an apprentice.

-In the French series, Greystripe is known as Greyfeather.

-When Darkstripe went to the Dark Forest, his parents tried to bring him to StarClan but Tigerstar chased him off.

-Cats use their tails to balance and have nearly 30 individual bones in them.