Continuity: In the scene immediately following Fiona's thrashing of Robin Hood and his Merry Men, Shrek is holding Donkey and they are staring at Fiona in disbelief. When Shrek drops Donkey and turns to follow Fiona, you can see his entire backside and it's unpierced. In the next scene, Fiona discovers an arrow in his rump that wasn't there before.

Factual error: Lord Farquaad wants to marry a princess to become a real king. The mirror shows him three princesses to choose from: Cinderella, Snow White and Fiona. But Cinderella wasn't born a princess! In fact, her only chance to become a princess was to marry a prince.

Continuity: Sometimes Shrek's front door opens outwards, at other times inwards, yet it can't be "two-way" because whenever it closes, it slams against the frame.