There are times when things, or people, or pets perform exactly according to their nature and yet the results are not at all as expected. Take the case of:

Family Dog

(whose Family went on a well-deserved vacation),

Teenage Girl

(retained as pet-sitter for the duration of Family’s vacation), and

Faithful Roomba

(the robotic vacuum).

Family departs on their week-long vacation with smiles on their faces, having arranged for Teenage Girl to drop in and visit Family Dog at least twice a day. Family Dog will be fed, watered, and taken for walks, during which the Deed will be done. Instructions to Teenage Girl have included words to the effect of “Don’t bring her back inside until the Deed is done!” Teenage Girl will also bring in the Daily Mail and Daily Newspaper, presumably observing during the course of these actions whether the Family Home is nominally intact.

Teenage Girl, being a teenage girl, promptly becomes distracted with her numerous daily activities and forgets that any arrangement exists regarding Family Dog, Daily Mail/Newspaper, or Family Home. She does not darken the threshold of Family Home even once during the duration of Family Vacation. Life is complicated, and there are many things. Shiny things.

Family Dog is a well-trained dog. She knows that the Deed is not done indoors. She knows that someone will come and take her outdoors, at which point the Deed will be done. But Family Dog is subject to her biology. She consumes the dog food that was left in her dish by the Family before they departed. She laps up the water that was left in her bowl. She is patient, but when the threshold is not darkened for an entire day, she relents. In several locations within Family Home. Family Dog is a good-dog-yes-she-is, but the Rubicon has been crossed. Once the Deed has been done indoors, well then.

Faithful Roomba is fully charged and prepared to vacuum robotically, to make its appointed rounds. It toddles around the house vacuuming up dust, pet hair, stray food crumbs, and the occasional paper clip. Or whatever else happens to be on the floor. Faithful Roomba does not question. It proceeds on its way with whirling brushes, scooping all comers into its compact waste receptacle.

Perhaps one day, in the bright future of home technology, a Faithful Roomba will be equipped with smart sensors and wireless technology. Perhaps, when it has assessed conditions in the home vis a vis its appointed task, it will have the sense to go up the chain, as we say, when something amiss is encountered. It may wirelessly contact the master of the home for further instructions, for clarity in the face of a daunting development. Future Roomba may say “Pardon me, sir, for the interruption. I have detected . . . well, an anomaly, sir.” (Future Roomba, I imagine, will have a refined British accent.) “Sparky has apparently elected to move his bowels in the kitchen rather than wait for his excursion outside. Shall I proceed as programmed, sir, or shall I contact the toxic waste service?” Or, at the very least, having detected the unmentionable, it will scuttle into a corner and wait.

But that bright future is not today. Teenage girl has gone about her affairs with the usual angst and inscrutablility. Family Dog is weakened from a lack of food and has retreated into a far corner of Family Home, a corner where Faithful Roomba does not venture. Faithful Roomba, alas, was faithful to the end, having toddled everywhere within its reach (and having left strong evidence of its extended sojourn), finally expiring next to the refrigerator with a last malodorous gasp.

Vacationing Family returns home surprisingly rested, refreshed, and utterly unprepared for the apocalypse that awaits them just across the threshold of their once-lovely Family Home. A pet-sitting career abruptly ends. A lovely dog will never be the same. A homeowner decides, after much discussion, that arson carries too stiff a penalty. And the local industrial cleaning service makes a tidy profit. All things have functioned according to their nature, yet the results are distinctly different than reason would have allowed possible. Beware the Edge Case.