Shocking your readers with something for which you’ve left them entirely unprepared often backfires. This is where little hints and teasers that plant suspicions in their minds should be used. Rather than distract from suspense, they actually add to it via anticipation, which is better than something out of the blue. If something too surprising occurs, it commits the primary author faux pas of throwing the reader out of the story.
Any jaw-dropping occurrences should thus have a foundation in credibility by being set up beforehand. Then fire away!