The first keyword listed for the movie "Everything, Everything" (2017) is a major plot spoiler. The keyword is "munchausen syndrome by proxy. As it's the first keyword, it's displayed on the homepage for the movie.
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IMDbPosted 2 years ago
Col Needham, Official Rep
bderoes, Champion

gromit82, Champion
Col Needham, Official Rep
Another example: there are 371 plot keywords for Titanic (1997) and one of them is lovers-reunited-after-death which seems harmless enough but it could be read as a huge spoiler after viewing the movie.
It happens even more with trailers. Trailer editors frequently include sequences from key spoiler scenes in the movie, yet out of context, they look totally harmless. Only when you re-watch the trailer after seeing the movie do you realize it has spoilers (the identity of the villain is revealed in this trailer for The Da Vinci Code (2006) -> http://www.imdb.com/videoplayer/vi3438280985 yet you would not realise without first seeing the movie so it's a non-spoiler spoiler :-). I only watch the trailer after seeing the movie so I am particularly susceptible to noticing this.