This slip up occurs regularly throughout the movie. Look at Ridley's hair. From the start of the movie, it looks a bit long and floppy. Then,

while Ridley is doing the maze to get the dragon eye, his hair suddenly looks shorter and neater.

Then soon afterwards, his hair becomes longer again.

Then, after Ridley has entered the cave which contains the special staff with which red dragons may be controlled, his hair once more looks shorter and neater. As soon as he leaves the cave and steps outside, his hair once more looks longer.

In the ensuing battle, his hair changes between and longer and shorter a few times, and after he enters the portal to return to the Mage's School, his hair becomes shorter and remains this way for the rest of the film.