C.S. Lewis

Nearly they stood who fall.
Themselves, when they look back
see always in the track
One torturing spot where all
By a possible quick swerve
Of will yet unenslaved--
By the infinitesimal twitching of a nerve--
Might have been saved.
Nearly they fell who stand.
These with cold after-fear
Look back and note how near
They grazed the Siren's land
Wondering to think that fate
By threads so spidery-fine
The choice of ways so small, the event so great
Should thus entwine.
Therefore I sometimes fear
Lest oldest fears prove true
Lest, when no bugle blew
My mort, when skies looked clear
I may have stepped one hair's
Breadth past the hair-breadth bourn
Which, being once crossed forever unawares
Forbids return.

~C.S. Lewis, Poems, "Nearly They Stood" (1933)