A block placed very near the player or another mob may vanish after a split second in Multiplayer.
Happens most often with non-cube blocks such as fences and repeaters, but with regular blocks as well.
Every time the problem occurred, the block was being placed on top of another solid block, directly adjacent to at least one other solid block, and always with what would appear to be plenty of room to place the block.
Happens when facing in any direction.
In survival mode, the block will leave the inventory, but picking up another item sometimes results in the number jumping back up to what it should be.
server is unmodded, client is unmodded.