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 thoug ...[click title to read more]...