There are various items which are missing from the API. An example would be
https://api.guildwars2.com/v1/item_details.json?item_id=23042
That should be Journeyman’s Salvage Kit, but instead it gives an error.
There are others, but I don’t have the ID’s handy at the moment. There are a handful of them that are in the https://api.guildwars2.com/v1/items.json list, but which don’t give a result.