It probably happens somewhere in the system every single day..but for the flight you are on it's pretty infrequent. The flight information printout that the purser gets tells him/her what movie to play... if they play the alternate movie it could mean a tape is missing or damaged. One time this happened to me years ago and I asked the purser about it and she showed me her print out and the print out itself was wrong compared to what Hemispheres said.