The mainhand item from another world visually appears in hand if you pause the game during world creation.

I left a world with a snow layer in my mainhand and then I created a new world.  I opened another tab while the world was "generating".  When I went back to the tab it was paused and I had a snow layer in my hand despite not having an item in my hotbar.  It was just a visual glitch because when i unpaused the game it disappeared.