Up till now my policy on spoilers was not to have them at all; to review and encourage people to watch a movie without giving anything away. I know first hand how spoilers can ruin your enjoyment when watching a movie or reading a book.
Once I was trying to decide which movie to watch at the cinema, so I went on IMDb to research. By mistake I clicked on Synopsis instead of Summary for the Spanish movie Timecrimes (2007). There was no spoiler alert. I still enjoyed the movie, however I didn't gasp at the plot twists, like everyone else in the cinema. I felt I missed on something, my viewing experience was diminished. By the way Hollywood is remaking this movie for 2012.
I reviewed some documentaries on this blog, and that was easy as there are no twists or endings to spoil. I also reviewed some movies, and I managed not to give anything away because there were no significant plot twists or unexpected endings. If there were I might have gotten away by saying for example the twist was "surprising" or "illogical", without actually revealing what it is. But I might have had to explain and reveal...
Another difficulty might arise when reviewing a sequel, especially if the ending of the original feeds straight into the plot of the sequel. In that case even the sequel's synopsis will contain spoilers for the original. These spoiler dilemmas are usually magnified for books, where there is more to reveal than for a movie, and the reader might not finish a book if he knows the ending.
Many times it's not possible to make a thorough review without commenting on certain twists, so now my spoiler policy is not to reveal anything unless it's absolutely necessary, and if I do, I'll mark it clearly:
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