Your reader was jolted out of the world you'd created and now had a choice - (a) shrug the shoulders and try to get back to the warm, cozy spot they were in OR (b) put the book down.
More than likely, depending on the jolt and what might have to be done - the option most often picked will be "B" and you, the writer, don't want that!
As a writer, you are creating a haven for your reader. It is a place where they can escape and be entertained, forgetting the worries of what needs to be done. A reader will flip pages, oblivious to the world about them until one of a few options happen:
You may be asking what I mean by jolting the reader out of the story. Let me give you a couple of examples, some are so subtle that even the best can be tripped up.
Example 1: It was high noon in the Nation's Capital as she dashed into Union Station. She strutted across the Main Hall, her heels clicking loudly on the linoleum floor. She gazed upward at the hideous gargoyles decorating the perimeter of the ceiling.
Example 2: He held his position, waiting, watching. Texas was a gunslinger's heaven. Behind one of those huge saguaro cacti he knew Bill Cassiday was hiding, waiting for him to make a mistake.
Example 3: Daisy stretched as she awoke and then smiled as she gazed out on her first morning in California. The Pacific Ocean shimmered as she watched the distant sunrise shed the clouds and ripple across the incoming waves.
Each of the above could have jolted your reader out of the story and made them question your writing expertise. I worked outside of Union Station and rode the train every day to and from work. Never once did I see any linoleum in Union Station. All the floors were marble or carpet in some of the nicer stores and restaurants that didn't want marble. But they didn't have linoleum. Also, I don't remember seeing any hideous gargoyles. They do have some faces and statues, but I would hardly call them hideous. I've traveled a lot in my life and been to Texas many times and love the cacti but again, the saguaro cactus is only native to the Sonoran Desert area which is basically in Arizona. Finally, I've yet to see a sunrise come out of the Pacific Ocean while visiting California. I've seen a sunrise in Hawaii, but then again, I was a lot further west. If you stand on the Californian shore, facing the Pacific Ocean, 99.44% of the time, I'm willing to bet the sunrise will be to your back.
Those were subtle but very obvious errors made by writers in stories I have read and/or edited. There are many other simple mistakes many writers make. Blood is pumped out of the body when shot and/or stabbed. It does not spread out like a seeping balloon. Horses can gallop fast but not all day. Think Pony Express. At full speed, a horse can go about 10 miles and then it will need to rest. Also, remember the rider is going to suffer during the outburst of a full gallop.
Today's television and movie drama has caused writers to lose sight of reality. A thrown knife will flip through the air so at that moment, there is a 50/50 chance of the blade striking the object. If that object is a human, the accuracy of catching the open area between the rib cage is even finer and hitting the heart, really pushes the odds limit.
By the way, there is more to flashing a sword around in a battle. I know because I took a couple of hours of sword practice with a person who does professional sword fighting at Renaissance Fairs. Until that moment, my story lacked authenticity during the battle scenes.
A simple fact can trip you, the writer, up. A simple misfact can jolt your reader out of your story. Be sure, never guess.