Almosts worthy of mention
Neither of these really count as silent penultimate panels, but they're bad enough in other ways to make them worthy of mention.
Cathy's penultimate panel is a scene-changing device. It let's us know that Cathy's mom is at a different location in the last panel. But it's entirely unnecessary. Dilbert has shown over and over again that we can follow a scene and time change without any signal other than different characters and a slightly different background. This sort of thing is something young and learning cartoonists do when they're not entirely confident of their graphic story-telling abilities. How long has Guisewite been at this?
And good ol' Mark Trail... Here's an example of drawing so fundamentally incomprehensible, a caption is needed to explain what the hell is going on. And even then, it's not entirely clear. But, of course, the appeal of Mark Trail is it's constant fundamental incomprehensibility. So this sort of thing just makes us love it even more.