A story where only one singular fantastical event happens around the end of act 2 but it's so Earth shattering to the characters it changes the genre, but is also the only fantastical thing to happen in the story
keys to a successful pivot. The first story has to be about as interesting as the second (preferably only slightly less) so that the reader is slightly miffed at the anticlimax but interested in what happens next. I'd like for the story to be otherwise mundane other than this one singular happening coming out of some other kind of other realm, a glimpse into a more interesting world. Perhaps the different pivots can tie in together in some way to paint a bigger picture of what is going on out there. It also has to be big enough to throw the original story off the rails a good amount.
A mayor of a small town has odd political leanings, but things are going well during his reelection bid until he gets assassinated(?) /Disappears (he seems to have been stolen by someone from somewhere else, but clearly supernatural). His workers know he was abducted but can't tell anyone and get caught up in the cover up and investigation as suspects
A private eye is investigating the gruesome murder of prostitutes in his area, he is closing in on the killer (and it's pretty clear to the reader who did it) An angel trumpeting the end times appears before him.
when a cult abducts him
When a ghost appears
He finds a mysterious object of unknown origin
He gets lost in an odd part of town