As I rather suspected would happen when I learned of the theme, to my mind there are a good many stories which are fine in themselves but don't really deal with the issue of liminality as I see it, but there were still enough left that I had to whittle things down to a shortlist.
Cat's Cradle -- The Harvest
Christine Wheelwright -- Metamorphosis
Elckerlyc -- If
emrosenagel -- Hair of the Dog
JS Wiig -- Into the Night
Luiglin -- Now
THX1138 -- Suspended in Twilight
Victoria Silverwolf -- Rebirth
After a good deal of cogitation and toing-and-froing, backwards-and-forwarding, where (liminally enough) I was in a state of terminal indecision I plumped for
Victoria's evolutionary tale.
Many ghostly ocean's edges thanks for the lovely mentions and shortlistings Christine, Victoria, Starbeast, AnRoinnUltra, Phyrebrat, CC (extra thanks for the special prize!), Hugh, Bren G and Luiglin!