The Transformation Ending

Gumball ends the argument between Penny and her family after his nested story made them realize they were essentially doing the same thing, but it's clear that his opinion of all of them has changed due to how they treated him and bolts out of their house the first chance they're distracted. Additionally the episode ends without the rest of Penny’s family coming out of their shells despite the name and plot of the episode implying otherwise, leaving their true forms unknown for the rest of the series.

Example of:
Bittersweet Ending