In 3.4, aiming a wand of cold balls at a potion mimic would simply destroy it: "The Gold Speckled Potion of Speed shatters!"
In 3.5-dev, this has been fixed: the monster becomes visible instead of getting wiped. But the message is still displayed: "The Gold Speckled Potion of Speed shatters! The Potion mimic freezes and shatters!"
Maybe display the message you get by revealing a mimic instead: "The Gold Speckled Potion of Speed was really a monster! The Potion mimic freezes and shatters!"
In 3.5-dev, this has been fixed: the monster becomes visible instead of getting wiped. But the message is still displayed: "The Gold Speckled Potion of Speed shatters! The Potion mimic freezes and shatters!"
Maybe display the message you get by revealing a mimic instead: "The Gold Speckled Potion of Speed was really a monster! The Potion mimic freezes and shatters!"