its a yellow potion (ooo powerful) and when merlin drinks it he finds out it was mustard. but you know, he thinks it was a yellow potion of power so he does better, then at the end berlins like "wheres my mustard". itd go sorta like this:
Merlin: *drinks potion* tastes almost like mustard. PREPARE TO DIE ENEMIES!
later at end of episode:
Berlin: Wheres my mustard? I left it right there under the 'yellow potion of power' sign!

Its like on space jam, where they drank the water but they thought it was special stuff so they did better. well not really like that