In his comic strip Lio Mark Tartulli often incorporates the unseen conventions of the genre into his humor, breaking the fourth wall in a way. (see Lio and the Fourth Wall for other examples).
In his 10/4/14 strip, Tartulli uses the panel border in an usual way (click on image to enlarge).
Panel borders are ever-present -- and never seen. Convention dictates that they're outside of the strip's action, in the same way that a frame isn't consider part of the image it's framing. Tartulli's gag is clever -- and by making us aware of something we take for granted, it's doubly so.