Continuity mistake: 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.
Continuity mistake: During the scene where Fiona pulls the arrow out of Shrek, Donkey faints. When he hits the ground about 6 leaves float up from the ground as if Donkey had fallen into a pile of leaves. When we next see Donkey laying on the ground and when Shrek picks him up, there are no leaves on the ground, they disappeared.
Continuity mistake: When Shrek walks out of his house and finds the fairytale creatures, you can see the three fairies go into one of the tents that were set up. The shot goes back to Shrek for a minute and then goes back towards the crowd and the fairies are back in the place that they were before.
Continuity mistake: In the scene where Shrek falls through the roof of the castle, in the shots where Fiona is in the background you can see the flower pot is on a table next to the pillow on the bed. However when Fiona reaches to get the flowers she reached down to the floor and not back towards the table with the flowers on.
Continuity mistake: When the magic mirror is brought in, we see the chain and hook that it is supposed to go on. Yet after the mirror is hung up, we pan around from the back of the mirror to the front and it appears that the chain has been attached to the back plate of the mirror. Where did the hook go?