Maybe I'm just a seriously flawed person, but I'm always looking out for the twist/ killer ending and so tend to look out for clues and spot the potential twists as I'm reading. Although I'm not always right, (oo, that's painful to admit!) I enjoy this aspect when reading. I can't think of any endings that completely floored me in the past few years, but I might if I mull over a bit longer.
However, when I was a sweet innocent I was shocked by the reveal at the end of The Turbulent Term of Tyke Tiler. I loved this book and thank Mr Freeman for being the teacher who introduced it to me!