I never got to finish the last eight pages of The Book Thief by Markus Zusak. I had rented a hut on a beach in Thailand and, on arriving, chose the thickest book on the shelf of the lounge's book exchange. I couldn't put it down and for the next 48 hours, outside of the occasional conversation on the beach with other travelers, that book and the beach were my only world.
Unfortunately I had misread my flight information and had fallen asleep the night before my departure thinking I still had the next morning to finish the book. On waking, however, I double checked and it turned out I was late. I thought about leaving behind one of the books I had just bought to bring back to Indonesia with me in exchange, but second hand English language books did not exist on the island I was living on and I didn't feel like lugging around a 600 page book just to finish the last eight, so I left it behind, unfinished.
This question itself reminded me of a documentary I saw about ten years ago called The Stone Reader. A book that remains unfinished, barely begun in fact, later becomes something of an obsession for the filmmaker. I recommend it as a must see for any bibliophile.