User blog comment:PlantyThePottedPlant/Random Topic Blogging III: Blog Harder/@comment-3006892-20121028200807/@comment-3006892-20121030215610

I shall explain:

In the comic "National Inventor's Day", in one frame, Phineas thinks something in his head that follows along with the conversation. Not say it, thinks it, it is clearly in a traditional cloud thought bubble instead of a speech bubble. Buford and Baljeet then reply to is as if he had just said it out loud.

Maybe an artistic error, maybe not, but it's canonified now.