For example, my husband and I are huge geeks, and we both work in computer fields. Every time we watch movies and TV shows with unrealistic depictions of computers/software/technology, we begin to play more attention to the inaccuracies than to the story the show or movie is trying to tell. I once read a romance novel in which two teenagers are friends, and one moves across the country with his parents. The other friend says there are no worries, because they can Skype all the time. Only one small problem: Skype wasn't initially released until eight years after the chapter supposedly took place. Why do I know it was eight years? Because I knew that these kids couldn't possibly Skype, and I stopped reading the book to fact check it myself.
When writing a story, make sure the facts back up even the small things you're saying. In my story "One Last Road Trip," I used Google Maps to map out my character's trip back home to Georgia, finding places along the way to have him meet up with various family members. I made sure the teams, schools, and jobs everyone had made sense based on timeframe, location, and character age. I didn't want a reader to lose the story because I'd put the wrong person in the wrong place doing the wrong thing based on reality and history.
Feel free to completely make up cities and towns. Invent new NFL teams or colleges. After all, it's fiction. But if you reference persons, places, or things that actually exist (or existed in the past), make sure that your facts line up with reality. Otherwise, you may have someone like me still telling people three years after they read your book that you made teenagers Skype before Skype was invented.