In singleplayer, when the player dies, entities like mobs, dropped items and item frames become invisible. When the player dies again, invisible entities become visible again.

While invisible, mobs cannot be interacted with in any way (punching, breeding, taming, trading, etc.), however hostile mobs can still attack the player, both directly (zombies) and indirectly (shot by arrows and fireballs)

Invisible mobs do still make sounds as normal, however they will not produce any particle effects (for example, the blaze does not make smoke)

Tile entities, such as chests and player heads are not affected by this bug.

Invisible items can still be picked up and pushed onto pressure plates

If invisible and visible entities are present at the same time, upon death, the entities that were invisible become visible, and vice versa.

Restarting the world makes invisible entities reappear.

Youtube video demonstrating this effect: 
https://www.youtube.com/watch?v=5JMOb7cSqFY