1. In Shrek 2, the Big Bad Wolf is killing time by reading Pork Illustrated.
2. In Shrek the Third, after Queen Lillian (voiced by Julie Andrews) headbutts two walls, she starts humming "My Favorite Things" from The Sound of Music and "Spoonful of Sugar" from Mary Poppins.
3. In Shrek 2, when "Shrek" wakes up in human form, a woman named Jill says she fetched him a pail of water.
4. In Shrek, Dragon breaks every part of the stained glass window in Lord Farquaad's church except for the dragon at the bottom.
5. In Shrek 2, you can see the king's bed and the murals on the wall allude to his time as a frog prince.
6. If that wasn't enough foreshadowing, the frog at the bar in Shrek 2 recognizes the king when he goes to visit Puss in Boots.
7. In Shrek 2, Fairy Godmother says, "What in Grimm's name are you doing here," a reference to the Brothers Grimm, who were the creators of many fairy-tale creatures.
8. In Shrek the Third, Fiona finds a hidden door by making her frog dad's statue kiss the horse's butt.
9. In Shrek, while he's trying to save Fiona from her tower, you can see a cookbook with a recipe for "Knightly Treats" that Dragon uses.
10. In Shrek Forever After, the tavern that Shrek's triplets have their birthday party at is the Poison Apple, the same tavern the villains hung out at in Shrek 2. In Shrek Forever After, you can see it's no longer evil and has been converted to the Happy Apple.
11. In Shrek 2, when the Fairy Godmother gets her wand back during the last scene and winds up to strike Shrek with a spell, Shrek pushes Fiona away so she won't get hurt.
12. In Shrek, Lord Farquaad has a portrait of himself in Sandro Botticelli's The Birth of Venus behind his bed.
13. In Shrek 2, the potion Fairy Godmother gives King Harold to put in Fiona's drink is a nod to "Love Potion No. 9."
14. In Shrek the Third, when Shrek interrupts the school assembly to look for Artie, the school band is playing "All Star" by Smash Mouth, the same song that opened the first Shrek movie.
15. In Shrek 2, when the cops are chasing Shrek and the gang, they reference him fleeing on a white bronco. This is a callback to the O.J. Simpson car chase when the cops were pursuing Simpson, who was fleeing in a white Ford Bronco.
16. In Shrek, Fiona gathers flies for Shrek to eat, then licks her fingers, hinting of her ogre background.
17. Finally, in Shrek 2, when Shrek infiltrates the Fairy Godmother's factory, you can see someone put a potion labeled "Viagra" on the conveyor belt.
H/T: /r/MovieDetails