Continuity mistake: When Westley and Inigo are duelling, Westley flings his sword so that it sticks into the ground and then athletically flips from a bar to where the sword is and nonchalantly reaches down and back and takes it out of the ground. But if you watch, he lands too far away from the sword to do this.
Continuity mistake: When Fezzik falls to the ground after fighting with Westley, there is a very large rock right next to him. However, when Westley tries to roll over Fezzik, the large rock is too close to do so. Therefore, the next shot shows them further away from the rock so Fezzik can be rolled over.