Holmes Minor

The original architect of 221 Baker Street haunts the edifice after the events of "The Mazarin Stone" (which, as with canon timelines, structure, voice, and everything else, throw a spanner into the floor plan), and like a malevolent sprite, the architect shifts things around in the night when everyone's and moves doorways just an inch or the table just a half-turn so that Watson, Holmes, and Mrs. Hudson are always bumping into things or second guessing their movements.