Anyone else remember that old artifact, 
Johydee's Mask? Did you know that in fact, it wasn't actually an artifact at all?
Well, sort of.
See, when TSR decided to put out their impressive 
Encyclopedia Magica (affiliate link) series, they had to figure out what to do with magic items that had the same name but were different enough from each other that they were effectively separate and distinct items. The solution they came up with was to append them with Roman numerals. So far so good.
But for some reason, in volume two of 
Encyclopedia Magica, the entry for 
Johydee's Mask from the AD&D 1E DMG (page 158), where it's clearly listed as an artifact alongside the likes of 
Heward's Mystical Organ or the 
Machine of Lum the Mad, was downgraded to an ordinary magic item. It had XP and GP values and everything (8,000 and 40,000, respectively). It was even listed in the random magic item tables in volume four (Table K), so this was clearly no mistake. Particularly since the AD&D 2E version of 
Joydee's Mask, from the 
Book of Artifacts (and listed in the EM as 
Johydee's Mask II), is still listed as an actual artifact.
Also, for some reason 
Encyclopedia Magica volume four only has an entry for the 
Wand of Orcus as it appeared in OD&D's 
Supplement III: Eldritch Wizardry, omitting the entry given on page 162 of the AD&D 1E DMG. I recall being quite ticked about that when I first noticed it, since I was reading the Planescape mega-adventure 
Dead Gods at the time, in which the 
Wand of Orcus plays a notable role.