@cstross
Imps embedded in your door decide who is allowed inside. Unfortunately easily fooled by a well-made oil painting. And the wizard added a back door: a turnip with a painted-on smiley face will open any door with this spell.

Almost certainly not-very-fatal imp-based home automation systems become all the rage for for the more gold-than-sense crowd.

A magical sword can predict and counter any attack. Gets disabled remotely mid-fight when the owner misses the monthly subscription.