When you switch Blocks on your hot-bar by pressing 1,2,3,etc... It would switch them but the Moment you place the Block Switched, it Turns the block you placed in your hot-bar into what was there before. So if you switch a Dirt Block with a Diamond Block and Place the Dirt Block, it would place it and then turn the item in your hot-bar into a Diamond Block, making 2 diamond blocks! Only tested in Creative...