Half-doors (wooden or iron) are 'invisible when placed. Hovering over the block reveals it's hit-box, but the texture is missing.

To recreate:
-start a new world (enable cheats, any game-mode)
-'/give <YourUsername> 64' (wooden)
or
'/give <YourUsername> 71' (iron)
-place the block

I guess these blocks are considered a glitch in themselves, invisible or not.
Still, it'd be nice to have them as a usable glitch.
(Sorry if this is a dupe thread, I didn't find anything relevant with the search)