I'm sure it was an innocent mistake, c'mon now, haven't you been reading this strip for years and years? Don't, no wait, Never attribute to malice what could be attributed to stupidity. Mr. Adams must have created TWO versions of the strip, one with the visible head explosion and one without the visible head explosion. Somehow both versions ended up on the path to be published. It's actually kind of a cool insight into the process of how ideas are expressed. I liked the "not visible" version more, it left more to my imagination.
p.s. and at least it's not an inflation joke....
p.p.s. doesn't "visible head explosion" sound like the name of new band? Oh wait, that was VAST, nevermind.