Revealing: In the scene where they're tumbling down the hill, Buttercup has a beard.

Continuity: 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.

Revealing: When Inigo and Westley are duelling, they are making athletic jumps. One does it after the other and when they land you can see the mat wrinkle under the "dirt" on the ground.

Other: When Westley and Buttercup are in the fire swamp and Buttercup is burned for the first time look carefully at the fire spurt, it's several feet away from Buttercup and is clearly too far away to have burned her.

Continuity: As Fezzik, Inigo, and Westley are developing their plan to storm the castle, Inigo says Humperdinck and Buttercup are getting married in a 'little less than half an hour'. It is bright and sunny outside. Next time we see them, before their master plan is carried out, it is dark, like it's the middle of the night.