This was a story about a skilled thief (and I think he was also a dog...but a humanoid dog) out to steal some quality jewelery from some castle residents. He decided to trick them with a modem; although he insisted to them that it was a working modem, he knew it was broken, and it could be safely plugged into the castle's ancient ethernet without fear of releasing a demon.
But a castle assistant, some kind of savant, realized the modem was broken, and suddenly fixed it after it was plugged in, ruining the plan and releasing a real demon. The jewels the thief turned out to steal were fake anyway.
Google hasn't helped me find this one so far...I'm hoping someone else might recognize it.