No set up
Three nicely colored silent penultimate panels this Sunday:
Bo-Nanas by Mark Leiknes
Out of the Gene Pool by Matt Janz
Get Fuzzy by Darby Conley
As Charles pointed out, the second panel in the Humble Stumble serves the same function as an SPP. It's everything but penultimate.
Is a punchline without a set-up still a joke?
The Photoshop artists behind the Pink Panther try to answer that question. And I believe the answer is no.
Is he following something, is he being followed, has he tried to get through the door before? Just a little bit of exposition would be nice. As it is, it's a gag with no reason to exist.