Not only was it the middle of November sweeps, but Timex, the main sponsor of the movie, was very concerned that, if "Heidi" didn't start at 7:00, the family audience would switch over to CBS or ABC.
NBC later claimed that their executives had decided, with 7 minutes to go in the game, to stay with it until it ended; however, the switchboard was already acting up, both because of people calling in to ask if "Heidi" would start on time and people calling in to ask if NBC would stay with the game.
Because of all those calls, they were unable to get through to their Broadcast Operations Control department, and so supervisor Dick Cline went with the written guidelines and started playing "Heidi" on the network at 7:00.
The executives finally got through, but not until the game was over. All they could do was have a graphic showing the final score superimposed over "Heidi" a couple of times.
NBC soon installed a telephone in BOC that didn't go through the main switchboard, for just such an emergency.
In 1985, NBC tried to deal with the Sunday night football overruns by producing several 15-minute episodes of the sitcom "Punky Brewster," the idea being that if a game ran a few minutes past 7:00, they could replace the scheduled half hour episode with a 15-minute episode starting at 7:15, and then the rest of their Sunday night schedule could start on time. In practice, however, the timing never quite worked out right, and the 15-minute episodes ended up airing later in the season, combined into a half hour time slot.