Continuity mistake: When Fiona shows her fighting talent by saving Shrek, they approach the trap holding hands, yet Fiona is not even visible in the medium range shot when Shrek is caught. (00:03:55)
Continuity mistake: When the door to Shrek's house is first opened (just after the honeymoon), it opens into the house. Every other time it opens to the outside. It's not a two-way door, because you can hear it hit the doorframe. (00:04:50)
Suggested correction: How is that a mistake? Lots of doors open both ways.
Doors that open both ways are called a double swinging door and use sprung hinges (or gravity) to bring it back to the center/closed position when not being pushed. It can go both ways because there is no stop casing for the door to hit against when closed (or frame is that is the design). We see Shrek's door hit a stop when slammed, and hear it hit the stop. Meaning there IS a stop, so it is impossible for the door to swing past it, meaning it is impossible to be a double swing, but we see it go in and out in different shots, which would be impossible.
Continuity mistake: When the request is made by the messenger to have Shrek and Fiona come to Far Far Away for the wedding party, as the trumpeteers troop off in the background you can see their footprints on the ground. However, when the same view is shown again seconds later the footprints are gone. (00:07:45)
Continuity mistake: At the dinner scene, Shrek says "better out than in" to Fiona. After that, the handwashing bowl and the shell's plate suddenly changes. (00:14:50)
Continuity mistake: During the first night in the castle, Shrek walks to the door of their room and his ears wilt. In the following pan shot to the king's balcony, his ears are standing up again. There is no time in which he could have voluntarily changed them. (00:22:40)
Continuity mistake: When Puss in Boots plants his sword in the ground then jumps out of his boots, his sword, boots and hat are nowhere near any trees or grass. Yet after attacking Shrek, Puss in Boots lands in his boots, and his boots, sword and hat have moved to being right beside a tree. (00:32:30)
Continuity mistake: When Shrek first meets Puss In Boots, Puss In Boots bursts out of Shrek's clothes "Alien" style leaving a tear. Then when we see them enter the Fairy Godmother's factory the tear is gone for the rest of the movie. (00:32:35 - 00:38:30)
Continuity mistake: When Fairy Godmother is throwing the books behind her, the book "Snow White" lands on the other books. "Snow White" noticeably changes position between two shots. (00:41:15)
Continuity mistake: When Puss is getting the Happily Ever After potion from behind the glass, the piece of glass he cuts out to pull the potion through disappears when the camera angle changes. (00:45:50)
Continuity mistake: When Shrek, Donkey and Puss all go to the barn to shelter themselves from the rain, the doors open inward. Later, after the potion has taken effect and Donkey's showing off his trotting ability, the doors open outward. (00:50:50 - 00:54:00)
Continuity mistake: When Fiona tells her parents she is going to look for Shrek, the hallway behind her is empty. Moments later when she goes to pick up her bag, it is shown in the hallway despite not being there moments earlier. (00:51:45)
Continuity mistake: When Shrek and everyone else are trying to get into the castle, shots of Shrek facing back toward the other side of the drawbridge are missing the others, i.e. the pigs, Wolf, Donkey, Puss, Pinocchio, and mice. (01:13:55)
Continuity mistake: Puss saves the Gingerbread Man from drowning in the castle's moat by catching him with his sword, however, he is on Donkey's back and the drawbridge is a couple of meters above the water. So the sword would have to have been 2-3 meters long to get down to the Gingerbread Man.
Continuity mistake: When Shrek, Donkey, and Puss are in the dungeon, Gingy and Pinocchio go down to save them. There is a leaf on Pinocchio's nose. After his nose extends and Gingy runs across it, the leaf is gone. Later in the scene the leaf is back.
Continuity mistake: When Shrek is reading Fiona's diary his fingers change position every time the camera changes.
Continuity mistake: When Shrek is transformed into an attractive human, his clothes are ill fitting. Princess Fiona wakes up and looks in the mirror and screams because she sees herself as a human. Fiona's dress fits perfectly in spite of her changed size.
Continuity mistake: When the King enters with the tea for Fiona and him, you see the cups ears are facing outward. He holds the tray with both hands. When he set it downs the one the king refers to as 'that's mine darling.' turns 90°, to face the king.
Continuity mistake: Fiona enters the study while her parents are drinking tea to tell them she's leaving. The doors are both wide open behind her and the hallway is completely clear. But when she leaves the room, she reaches down and picks up her suitcase in the hallway, which would have been visible behind her moments before.
Continuity mistake: When the cooks arrive with the food during the dinner scene they set the food down on the table, but in the next shot from above they are still walking towards the table with the food.
Continuity mistake: When Shrek and Donkey and Puss are in the potions room, they are stealing a potion which is called Happily Ever After. Shrek tells Donkey to keep watch, the door is ajar in the next shot it is wide open.
