The owners' loss stemmed from that unusual occurrence.

Now, a catastrophic blaze, once an unusual occurrence, could be set off by the heat from a lightning strike.

That was not an unusual occurrence.

And was such an unusual occurrence really rather enjoyable, a novelty, a challenge?

There was great tension surrounding the game, not least because of the long-standing rivalry between the two neighbouring nations, but this dissipated over an unusual occurrence just before kick-off.