Other mistake: When a glass of milk is poured, if you look closely, the glass is filled but the level of milk in the bottle doesn't seem to lower. (00:16:00)
Other mistake: Princess Fiona is cooking the eggs on the rock that is over the fire. She uses a big green leaf as an oven mitt to protect her from the heat, but then places her other hand on the rock, without a leaf, and somehow doesn't get burned.
Chosen answer: Who is "everyone"? The only person surprised he can talk is the one human who is processing the fairytale creatures for banishment, and this was used as a plot device to highlight a joke (between him flying and talking).
MovieFan612 ★
Fiona also says "it talks", after he "saves his ass."
To be fair, she was locked in a castle for the majority of her life, so she wouldn't know that donkeys would be able to talk.
And yet he wasn't the only one surprised by it; watch the scene again and Fiona was surprised, too.