mischance

By some mischance, a police car passed by just as the alarm went off, so the burglars were caught in the act.

The cast of Mischance - 2014 includes: Dean Curtis as Officer 2 Ben Leasure as Ryan Gardner Hardy Winburn as Clancy Carter

The opposite of having an opportunity would be a lack of one.

The opposite of opportunity (beneficial situation) could be misfortune, mischance, or dilemma.

George Redway has written:

'A chapter from the book called The ingenious gentleman Don Quijote de la Mancha, which by some mischance has not till now been printed'