haply
by chance
by luck

It's only by luck that we know where the first kill zone is.

By sheer bad luck he would trip over them no matter what they did.

But all of a sudden I stumbled on the very thing, just by luck.

By luck or design the big pots had all gone to their captain.

"What if someone does something good just by luck- and can't ever do it again?"

By good luck he hit the bigger boy on the mouth.

Any shot which struck home did so by pure luck.

Then we picked it up again, just by luck, a half hour later.

It was only by great good luck that she stumbled upon an older married man who took an interest in her.

"By luck you must have touched a pattern which was not one set for defense."