Daenerys speaks High Valyrian fluently, as commented in And Now His Watch Is Ended (2013) and Second Sons (2013). She should have been aware that the High Valyrian word "prince" is gender-neutral, without being told by Missandei.
Jon says that dragonglass can destroy both the White Walkers and their army (namely the wights). As Sam discovered in the third book, dragonglass is lethal to the White Walkers, but is ineffective against wights.
Ashemark is misspelled as "Ashmark" on the Painted Table.
Sam tries to numb Jorah's pain with rum, even though milk of the poppy is the standard anesthetic in Westeros; Sam should have known since even non-maesters have referred to the substance several times before.