Re-creation:
If you have a world where you are in creative mode (not sure if it needs to be a creative world from the beginning or survival will work if cheats are enabled and one switches to creative, but I'd imagine it's a creative world only) and change to Spectator Mode, leaving the world and coming back in places you in a partial Spectator Mode, mixing both creative and spectator. Below are the features that stick from either mode:

Creative:
-Flight, but not able to faze through things
-Crosshair Sticking (read below)
-Fully Visible Body
-Ability to Pick Up Items

Spectator:
-Ability to Merge with Entities (but Crosshair Sticking Bug)
-Flight (but Creative Mode version – no fazing)
-Survival Inventory

Crosshair Sticking:
-For whatever reason, the crosshair temporarily remains with some entities upon merging with them (this isn't present when using standard spectator mode, however). The sticking involves the crosshair remaining onscreen for a short duration, permanently, or not at all. Left-clicking while it's on screen and also being in any third-person view (regardless of whether the crosshair is over the entity) crashes the player with the attached image's error (Attempt to attack an invalid entity, I believe). I have tested with many, but not all, entities and listed which have permanent, temporary, or no sticking.

Entities (Going By Internal Names, per /summon):
Crosshair Sticks:
-Zombie
-Creeper
-Spider
-Sheep
-MushroomCow
-Giant (quirky; sometimes crashes, sometimes removes you from spectating them)
-MinecartHopper
-Boat
-
Crosshair Briefly Stays:
-Skeleton
-Bat
-Villager
-Slime
Crosshair Disappears (Probably Proper Functionality):
-Endermen
-PigZombie
-WitherBoss (mostly; others, it's brief)