Description :
Hoppers don't work properly if they contain 2 types of item.

What happened :
Let's put items (emeralds) in the hopper on the right.

• If the bottom hopper is empty : emeralds go in.
• If the bottom hopper has 1 slot with emeralds and 3 slots with snowballs : no problem.
• If the bottom hopper has 1 slot with emeralds and 4 slots with snowballs : it doesn't work anymore and all emeralds go in the chest.

So basically it breaks all the hopper filters players have made.

More screenshots and explanation :
From Fernando MK : http://imgur.com/a/BXF4L