It's very rare that I just stop watching a movie - usually I try to finish it, unless it's just awful.
Last night I made an attempt to watch The Lady in the Water, and hated it so much that I turned it off 20 minutes in.
I'm still mad that I lost that 20 minutes. I'm even angrier than when I saw Austin Powers 2. At least I kind of felt entertained (although gaudily) by that one, even though it was stupid.
I'm not sure I would be able to articulate what I hated about it. I just hated it.