Item ID 0x00, if held by a Pokémon, causes the game to behave as if the Pokémon is not holding an item. In the international releases of Crystal, it crashes the game due to an oversight.
Item IDs 0xC2 and 0xC3 both contain an item named "TM04". 0xC2 is the real TM04 while 0xC3 is a unused and useless item that shares the same name, is stored in the Items pocket of the Bag, and has no "USE" option. The same applies to item IDs 0xDB and 0xDC, in which both are "TM28". When held by a Pokémon, the name of both unused TM04 and TM28 is instead displayed as Berry (Japanese: きのみBerry). Otherwise, these two dummy TMs behave like Teru-sama.
Item IDs 0xFA-0xFE have the name of HMs but they are stored in the Items pocket of the Bag and behave like Teru-sama. In addition, 0xFA to 0xFC are internally used by Crystal while displaying the list of moves taught by the Move Tutor.
All these items can be sold for 19660, just like any Teru-sama.
Item ID 0xFF is similar to its Generation I counterpart in that it does the same thing as the Cancel option and hides items below it. However, it does not trigger the USE/GIVE/TOSS/SEL/CANCEL menu, it cannot be held by a Pokémon, and it cannot be purchased or sold.