I cast Repulsive Mutation, X=1, with an opponent's Topiary Panther on the stack. I had a 3 power creature, and they had 2 mana open. I clicked my creature, clicked the Panther on the stack... and nothing. My opponent paid no mana, there was no red "wub wub wub" notification said the spell couldn't be countered, it simply resolved.

My best guess is what happened is that the client failed to register me clicking on the Topiary Panther and simply... did not warn me, which caused me very directly to lose the game. This shouldn't happen, for multiple reasons.

If whoever is reading this has the ability to send bug fix/feature requests to the developers, I would really like you to send a request asking for a warning prompt if you aren't targeting a spell with Repulsive Mutation with an opponent's spell on the stack.