Bedrockgames
I post in the voice of Christopher Walken
I would also agree that the DMNPC can be a issue, but to me it is just part of a bigger issue. Sometimes the party completely screws up your plans (not necessarily a bad thing) and the hardest thing for a DM to (at least for me) is take a deep breath and take it on the chin and then find a way to roll wth it. Sometime this requires you to tell the group you need half an hour to rework some stuff --- sometimes it means it becomes a board game night.
One of my big realizations as a GM was that it is perfectly fine for players to beat an adventure in ten minutes if they figure out a way to do so, and battles with the big bad guy don't have to last dramatically long (one of our best final battles invovled the PCs teleporting into the villain's court, killing him in one hit and leaving).