"Don't Start Un-Believing" is a song Mabel sings in the episode "Double Dipper" after she demands the '80's-est crowd-pleasing-est rock-ballad-iest song you got!' The song was used by Mabel to attempt to beat Pacifica Northwest in the competition for the Party Crown.
Don't start un-believing
Never don't not feel your feelings
I'm gonna do a flip!
[Mabel attempts to do a flip]
That was for you guys!
- It is a parody of the Journey song Don't Stop Believing.