1 {scene: the entire four panels of photos and the text area above them is filled with a beige colour}
4 Me: (unseen under the blanket of beige pixels} Bloody software! What do you mean, "Can't undo flood fill"?

This actually happened to me once when making a strip. Fortunately my software does have an undo feature that works, but if it didn't you might have seen this one earlier...

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2013-05-08 Rerun commentary: Flood fill, for those who are unfamiliar with the term, is a computer graphics processing operation in which connected regions of the same or similar colour are replaced by another colour. In a typical use, the user will click on a point in the image, and all pixels of the same (or similar) colour connected to that point are changed. For example, if you click on a point in a digital photo in a blue sky, all the pixels of blue sky might be changed to red (or whatever your chosen replacement colour is), while the other pixels are unaffected.

Typically, graphics programs (such as Photoshop) have a "colour tolerance" parameter which the user can set. If the tolerance is 0, then only pixels of exactly the same colour as the clicked pixel are filled with the replacement colour. In the blue sky example, the surrounding pixels, while close in colour, are unlikely to be exactly the same, so the fill won't spread very far, if at all. But if the tolerance is set to something like 20%, then it might pick up all the blue sky, but not the white clouds or the green grass. If you raise it to, say, 50%, it might start colouring in the white clouds as well. And at settings close to 100%, the flood fill will affect almost everything in your original image.

Wikipedia has an article on flood fill, but it's currently a bit too technical, including algorithms for how to implement flood fill on a computer, but somewhat short on a simple explanation of what flood fill actually is.