Your back-building of coincidence is essentially Chekov's gun, viewed from another angle. A potential downside is that if it isn't very artfully placed, the reader will see the significance before the characters, and grow frustrated at their idiocy.
Thanks for the comment. Yeah, you're right, it's just something you can come up with researching tv tropes. Basically it's like the story teller's technical manual, and again, being annoyed at stupid characters will be something you fix by reading tv tropes.
In some fringe cases you want the reader to "get the idea" before the characters do.
One reason is to annoy them and create stress, which is often used in thrillers, where you can see exactly the character's going to die.. or are they? Yep, thrilling.
Another one is to give the reader a feeling of "superiority", in the meaning of seeing a grander scheme of things. By making this level of reasoning fairly predictable you can use this as fundament and build more complicated meaning on top of it.
