If you are in spectator mode and then decide to disconnect and then reconnect to your world, you can't pass through entities.
Here is a gif to show you the bug (couldn't attach it because it was too large) : http://giant.gfycat.com/SmugSomberCanadagoose.gif
Can confirm, also occurs in multiplayer. Keep in mind that blocks are not entities, but you are able to interact with the world as you normally would after you've reconnected, meaning able to pick up items and push entities.