1. Wrong: Walking with his head down, the ladder still smacked Julien.
Better: Walking with his head down, Julien still smacked himself into the ladder.
Explanation: The first part of the sentence must describe the word immediately following the comma; however, the ladder isn't walking with his head down. Julien's name must be relocated to the spot after the comma and other changes must follow accordingly.
2. Wrong: We loved looking at the new outfit on the baby, which had a cute flowered pattern.
Better: We loved looking at the cute, flower-patterned outfit on the baby.
Explanation: The which clause must describe the words immediately preceding the comma, but the baby doesn't have a cute flowered pattern.
3. Wrong: If you want to never have to go to detention, start by getting enough sleep.
Better: If you want never to have to go to detention, start by getting enough sleep.
Explanation: In many languages, an infinitive uses just one word. While infinitives use two words in English (to and the verb part), the custom is not to put any other words between those two words. The error is called a split infinitive.
4. Wrong: In Wang Wei's and Jose's brand new outfits, Maxime thought they looked great.
Better: Maxime thought Wang Wei and Jose looked great in their brand new outfits.
Explanation: The word order of the original sentence implies that Maxime is wearing the other people's new outfits.