While writing, there are certain tools and traps that feel much more apparent and perilous (respectively) when writing with feedback/critique in mind.
Chekhov’s gun is an example of a tool, and it’s in the vein that I mean to discuss. “One must not put a loaded rifle on the stage if no one is thinking of firing it.” Usually I think of it in relation to including only necessary elements, but here I mean it for its allusion to foreshadowing.
If your main character mentions that he always wears am amulet for protection, then have that amulet protect him at some point. Be it pivotal or casual.
Examples: The assassination target loves hunting, he is killed in a staged hunting accident. A window is left open, this is used as an entry point.
Then there is coincidence. This is not, although one might mistake it for, a trap. Coincidence can be an element of realism. A coincidental event or fact can lead to something important, but doesn’t need to have greater meaning.
Examples: The main character meets a stranger who becomes a friend. A dog known for digging finds an important artefact. A teacher meets a troublesome student’s parent at the grocery store.
The reason that coincidence can trip one up is because it can very easily look like or even become contrivance. An easy way to spot a contrivance is to check if you can seriously (not through an injection of cynicism) describe it using the phrase, “It just happens that/to be”.
Examples: It just happens that the girl the hero met is the princess. The main character’s best skill just happens to be one that the ice giants respect above all others. The main characters get lost and accidentally find the missing general.
Contrivances don’t have reasons and cannot really be explained as a neutral coincidence. Coincidences that become important tend to work forward. A seemingly unimportant event turns out to later have significant meaning. But contrivance is a tool of a lazy writer. They work backwards. The writer needs to have a mentor to train the main character so the old man in the pub walks up to him.
For all that they do technically have a reason–usually that reason is “this is how the story is supposed to go”–they can often be described with the phrase “for no reason”.
It’s difficult not to try to use Chekhov’s gun’s conservation of detail exclusively and try to avoid coincidence in fear of contrivance. But sometimes, serendipity does occur and it can be a fun element to introduce change.