This bug / change in game rules breaks the most common hopper mine-cart mob-drop collection system in survival. For such systems to be at all reliable 1 high slimes / magma cubes must be prevented from spawning on the tracks. Tracks can not be placed on transparent blocks, and if transparent blocks are placed above the tracks, 1 high slimes and magma cubes still spawn, get stuck and completely foul the system. The conventional solution has been to place solid blocks above the tracks with two snow layers on the solid blocks. This had the affect of suppressing spawning at both track level and in the item drop zone while still allowing the items to be picked up by hopper mine-cart.
Allan Rogalsky
Is duplicated by MC-53568
Confirmed in my 1.8 snapshot test world.
This bug / change in game rules breaks the most common hopper mine-cart mob-drop collection system in survival. For such systems to be at all reliable 1 high slimes / magma cubes must be prevented from spawning on the tracks. Tracks can not be placed on transparent blocks, and if transparent blocks are placed above the tracks, 1 high slimes and magma cubes still spawn, get stuck and completely foul the system. The conventional solution has been to place solid blocks above the tracks with two snow layers on the solid blocks. This had the affect of suppressing spawning at both track level and in the item drop zone while still allowing the items to be picked up by hopper mine-cart.