I was about to kill a few zombies in a cave, and while scrolling to select my sword. I tried to place some torches real quick, the placed, but then disappeared. The torches were in my off-hand. After killing the mobs, I had to figure out why I couldn't place those torches.

What I found was when you have an empty main hand, and try and place a torch in your off-hand. the torch goes down, but disappears from your inventory and where it was placed.