When the wizard teleports to the mountaintop for the final battle, thunder continually rumbles under a clear sky, even though the wizard hasn't yet magically summoned the electrical storm (which he will do in a few moments).
The final battle with the dragon takes place during an eclipse and a magically-summoned thunderstorm. When the dragon rises above the cloud cover for its final attack, it's in full sunlight.
During the solar eclipse takes place, the moon slides in front of the sun, from below. In real life, the moon moves from right to left (northern hemisphere) or left to right (southern hemisphere).
In the dragon's lair, a wizard sees the dragon reflected in the water as it rises up behind him. Only things in front of or above a viewer would be reflected in a flat surface. He might hear water dripping from the dragon, but he wouldn't see the dragon's reflection.
In the Paramount Widescreen Collection edition, during the lottery scene the man drawing the tiles reads the name "Princess Elspeth Ophelus; Filia Regis," Latin for "Daughter of the King". The subtitles say "Clearly outrageous".
When the first girl is chained up for the dragon, she wrenches her hand free from the shackles. The soft padding inside the shackles is visible for a few shots. When the shackles appear later on, they are plain metal.
Obvious matte work during some of the flying sequences and when the dragon is moving on the ground.