Halley (massive spoiler alert)
The ending would have been far less happy if the film had shown her in tears, jilted at the aisle, 10 minutes before we see Jon and Sara reunite at the ice rink.
Don't get me wrong, I think most of the plot was extremely clever, but it's harder to feel happy for them knowing how much hurt they'd caused an innocent person...who ironically brought them together when she bought that second hand book.
Of course, they may have caused her even more hurt if the marriage had gone ahead, but she'd probably never know that.
London Marathon for AIDS patients: www.justgiving.com/icdcmarathon