Even with a relatively short 16-game regular season, it is virtually impossible to go through an NFL season undefeated. There are only two teams in league history that finished the regular season without losing a single game. The 1972 Dolphins did it (on a 14-game schedule) and the 2007 Patriots did it. That's it.
The Cowboys lost yesterday. It may have been an unexpected loss, it may have been an unfortunate loss, it may even have been an avoidable loss, but at some point the Cowboys were going to lose games. And not just one.