This bug allows a player to be on the Minecraft menu or single player world and have other players see them in a realm. In a singleplayer world this causes realms chat to show up in singleplayer, causes problems with the singleplayer day/night cycle, and loading a singleplayer world.

To cause this bug, attempt to join a realm and then click the "Cancel" Button when connecting to a realm until someone on your realm sees you join but not leave even though you returned to the main menu of Minecraft.

The repercussions of this bug if a player tries to play a singleplayer world is that the realms chat will appear in your game and it seams like every other frame the sun and moon will bounce between the time in the singleplayer world and the time in the realm. The single player world also may not render the blocks after loading the world. I have attached a screenshot of my singleplayer world that I discovered this bug in. On the realms side of things, other players will see your player floating 1/2 block above the ground and only see a join message and not a leave message.