When you place a block while standing in that place, it gets consumed. If you try to place the same block where it vanished, it gets back into your inventory and the other stays on the ground. Tested for survival mode only.

What I expected to happen was...:
When placing a block, the block appears where it was placed.

What actually happened was...:
When placing a block, the block flashes in place and does not get placed. The block gets removed from inventory and some "fiddling" with inventory (place another block, place a torch, etc) sometimes makes the missing block reappear in inventory.

Steps to Reproduce:
Place a block at your feet
Stand as close to that block as possible
Attempt to place a block on top of this block, at your head level, and it will disappear.

Additional Information:
The same effect is observed when placing a block at foot level, it is just easier to reproduce when there is a block at foot level to ensure the player is not occupying the space where the block will go.

In 14w32b this happens when facing North, East and West, but I was not able to reproduce it facing South.
In 14w31a this happens when facing North or South, but I was not able to reproduce it facing East or West.