I mention this as coincidence number one.

The following evening Middleton, having by chance left the jewels at home, dines, and goes to the theatre by appointment with Carr.

Unique cab accident occurs, in which Middleton is knocked on the head and rendered unconscious.

Coincidence number two.
Miss Middleton's house is broken into that same night on Middleton's return to it.

Coincidence number three.