If you've done a developmental edit with me, chances are pretty good that in your Editor's Letter, there's been a comment or two (or three) in regards to coincidence. This is a big thing for me, and usually something the reader catches onto, even if they don't know *exactly* what is bugging them about a story.
What is coincidence?
A remarkable concurrence of events or circumstances without apparent causal connection.
All writers have done this. Gotten to a part in a story where they need a catalyst of some sort. A character to come in and engage with the protagonist. Tension because the plot has slowed a bit. A hobby that the secondary character needs to love so there's a change in scenery.
Whatever the case, we come up with something that the story needs and it seems to come out of nowhere (because it kinda did.)
The reader can sense this.
Usually it's not difficult to get rid of this feeling of coincidence. It can be as simple as placing that 'random' character in a scene or two earlier in the story. Planting that seed of tension during the first few chapters under the guise of something seemingly mundane. Or having that secondary character mention his desire to (fill in the hobby of choice) in passing.
Ultimately, even if we know to watch out for coincidence as we're writing that first draft, it will sneak into our work sometimes. And that's what critique partners and editor's are for. They're someone who is not as close to the work, the characters haven't lived in their heads for months, and can look at the story with fresh eyes, pointing out the things that seem to 'come out of nowhere'.
So as you're writing that first draft, think about what you can do to avoid coincidence.
And then ask your critique partners and editor's to look over your work, and make sure those buggers didn't get in there somehow still.