All of the World Series facts were true in this film. The Mets Cleon Jones really did get hit on the shoe with a pitched ball. At first the umpire didn't award him first base but when Mets Manager Gil Hodges showed him the shoe polish on the ball the umpire changed his call and awarded him first base. It became known as the "Shoe Polish Incident".
The "time travel" mechanism of the film, by which only messages can be sent through time, is similar to the method used in an award winning novel, "Timescape" from 1980, written by Gregory Benford and Hilary Benford.
In the movie, Frank Sullivan was an FDNY firefighter, and the numerical address of the Sullivans' house is 343. One year after "Frequency" was released, 343 FDNY firefighters died in the collapse of the Twin Towers of the World Trade Center on September 11th, 2001.
Elizabeth Mitchell plays Jim Caviezel's mother. In reality, Mitchell is younger than Caviezel. Similarly, though the story and Mitchell's portrayal suggest that her character is about the same age as her husband, played by Dennis Quaid, she is 16 years younger than Quaid. At the time of the film's release, Quaid had just turned 46, Caviezel was 31, and Mitchell was 30.
In 1997 Renny Harlin was going to direct the film with Sylvester Stallone playing the lead, but Stallone asked for too large a paycheck and eventually neither Stallone nor Harlin was involved in making the picture.
Elizabeth Mitchell later starred on the popular television series Lost (2004) as Juliet in which one of the main characters was named Jack Shepard. In Frequency, Shawn Doyle plays a character named Jack Shepard, and later had a guest role in season 5 of Lost: Eggtown (2008). Both show's recurring themes included time travel and alternate histories.
Other strange 9-11 trivia: Besides the coincidence of the Sullivan home house number being the number of NYFD lost, two of the doomed flight numbers appear. Frank's fire truck is (United) 93 and his son Johnny wears a jersey with the number (American) 77.
Brian Greene: The film's physics consultant appears early in the movie as himself, being interviewed by Dick Cavett. There are two scenes; one with old footage of Cavett interviewing Greene in 1969, then a scene with an interview in 1999 with contemporary footage of Cavett and an "aged" Greene.