Be Aware of Reel Changes?
Many books on film scoring mention to avoid having any music over reel changes. Not too long ago when most film projection was done analog (meaning to actually have big reels of film being put into projectors), films were broken down into several reels.
Usually a reel could hold around 15-20 minutes of film. After this time had passed a second projector needed to take over exactly at the moment when the first reel was done to project the next reel and so on. Between these reel changes, it was quite common for the audio track to have occasional pops, clicks, crackles etc so any music cue that would overlap would have noticeable audio glitches. Reel changes were planned quite early on in the production of a film so it was very often taken care to have a reel change on a long black cut with practically no sound to avoid any glitches.
With digital projection taking over, most cinemas are equipped with such a projector nowadays being able to show a movie as one constant stream. So there are no reel changes anymore, hence no more limitation regarding the overlaping of audio/music. Still, many movies are broken down into smaller portions in the working process (easier handling etc.). This is usually done by Acts now, so following the story line. So the bottom line is: in most cases you don’t need to worry about any such things anymore today. Only if there is a very small chance that the movie you’re working on will be projected analog, you might take this into consideration again.