The bug

Some outlines on item slots are embedded on their respective GUI texture, instead of being a separate texture that is dynamically added if no item is in the slot (as is currently done with armor and the off-hand, for example).
This causes the item outlines to remain after an item was placed in the slot; therefore, an overlap may occur if the item does not match the outline exactly, which is, for example, the case with splash or lingering potions in the potion slots of a brewing stand.

Affected GUIs

• brewing stand
  • fuel slot
  • potion slots
• enchanting table
  • lapis lazuli slot
• horse
  • saddle slot
  • horse armor slot
• llama
  • carpet slot
• smithing table
  • ingot slot

Steps to reproduce (brewing stand)

1. Get a brewing stand
2. Put a normal potion in one of the slots
   → You can see no outline
3. Now put a splash/lingering potion
   → Part of the slot outline is still visible

You can do the following to verify that, for example, the armor slot outlines work differently:

1. Get some armor
2. Open your inventory and place the armor item in the respective slot(s)
   → All the outline disappears, even if the piece shape doesn't cover it completely