When you have 2 or more stackable items in different slots in your inventory, the amount of items goes over the black text box.

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Add at least 2 different (or same, as long as they are in different inventory slots)stackable items into your inventory in either survival, adventure or creative mode. You may have as many items in that slot as you want.

2.  Hover over one of the slots with the item(s), so that the black text box appears over the other stack of items.