Thanks @Rainbow_Dash_4_real for the first fact!
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-In The Sight on page 178 it says 'have' instead of 'has': "What if StarClan have no answer for you?"

-It tore Kate apart to write Turtle Tail's death, she says. She also mentioned that she imagined it was a Ford Model T that ran her over.

-In The Ultimate Guide Mapleshade is put in the RiverClan section even though she has never been officially a part of that Clan.

-Vicky thinks Greystripe's kittypet name was Stripes because Greystripe was based off of a childhood pet with the same name.

-Boulder showed BloodClan to Tigerstar to impress him.