If you smelt an item and precisely as it finishes smelting, you shift click it out, it creates a visual glitch which appears to consume the item that had finished smelting if you already have that item. The same thing happens with brewing stands except weirder. Instead, you wait for a potion to almost finish and right as it does, you shift-click in the next ingredient when you in a different inventory slot another of that item. It appears to make the brewing of the first potion restart, even though it successfully puts both of that item in. For example, you are making Fire Resistance from Awkward Potions. Right as the 3:00 potion finishes, you double-shift-click a single redstone dust in when you have a separate inventory slot also with a single redstone dust. It will appear to restart brewing from Awkward Potions even though they are actually Fire Resistance Potions. If you remove the "ghost" magma cream, you will see that it is actually 2 redstone dust and the 2 redstone dust in your inventory will disappear.