After the first gargoyle attack, the doctor asks his daughter to play back the cassette recording. The playback reaches the point where she had to flip the tape over during recording and continues playback without the tape being flipped.
After the Gargoyle takes Dr. Boley into the cave and they are talking the dubbing doesn't quite match the Gargoyles lips. And after he tells Boley "Oh, you'll be around to see it (the end of mankind)." the dubbing stops and you can hear Bernie Casey in his real voice say "Boley."
In the scene where the police have arrested five bikers and have them lying on the ground, the sun is visible in the sky behind them, yet shadows can be clearly seen as though the sun were coming from behind the camera.