The unique shader that is applied when a player spectates an Enderman, spider, etc. gets stuck on if the player is killed using commands. The player is no longer spectating the mob and can freely move. The only way to remove the shader is to spectate a different mob that does not have a unique shader like a sheep, cow, etc. or reload the world.

The shader will be stuck on even if the player leaves spectator mode and enters survival or creative mode.

This is very minor, but I could see it being a problem if a server hosted a minigame involving spectating mobs and the player is killed when the minigame is over (I don't know if this problem occurs on servers, though)