1. At their first battle of the bands gig, Scott wears a Plumtree shirt. Plumtree is a Canadian band that has a song named "Scott Pilgrim," released in 1997. This song inspired Bryan Lee O'Malley to write the Scott Pilgrim comics, which were the source material for the movie.
2. There are five posters in the movie for fake Lucas Lee movies. Four of the movie titles refer to Plumtree songs. Additionally, the names in the credits on the poster are the names of various Scott Pilgrim production crew members.
3. Here's some insane attention to detail from the effects team: When Scott pees in the bathroom, the "pee bar" is reflected in the mirror.
4. Ramona mentions that Todd punched a hole in the moon for her. You can see the crater it left whenever the moon appears throughout the movie.
5. When Scott tosses away the Clash at Demonhead CD, it lands on top of a bunch of Beck CDs. Beck wrote the music for Scott Pilgrim Vs. The World.
6. Early in the movie, Sex Bob-Omb uses "Lame Brand" amplifiers. Later, when performing at Gideon's, they're using "Sweet Brand" amps.
7. When Ramona is explaining the Seven Evil Exes to Scott, all the lights behind her are in X shapes. Once they agree to date, all the lights behind them are shaped like hearts.
8. When Scott is talking to Envy, you can see some photo booth pics of them behind Scott. One of the photo sets shows Envy getting up and leaving Scott alone in the booth.
9. Knives ~literally~ has stars in her eyes when she sees Scott perform with Sex Bob-Omb for the first time.
10. This poster in the coffee shop window — which appears for only a second or so — is a little nod to people who are looking for details in the background.
11. There are hidden Xs throughout the movie. In one shot, while Scott is pondering the Seven Evil Exes, there are a total of seven X signs behind him.
12. After Scott has defeated Roxy (number 4 on the list), he wears a modified Fantastic Four shirt with "4 1/2" on it. This indicates that he's between numbers 4 and 5 on the list.
13. When Scott asks for a list of Ramona's exes, she jokingly says that she'll have it laminated. Later, she gives Scott an actual laminated list.
14. Roxy's weak point is the spot behind her knees. She also happens to have holes in her stockings in that same spot.
15. After Scott's battle with Todd, his bass is destroyed. Later, he's still using the same guitar, but it's duct-taped back together.
16. The song "Black Sheep," which the Clash at Demonhead perform in the movie, was written by the band Metric. Bryan Lee O'Malley said in a Tumblr Q&A that he used photos of Metric singer Emily Haines as inspiration for drawing Envy Adams.
17. Every Evil Ex that Scott fights has a reference to their number. When Matthew Patel appears, Wallace calls him "that ONE guy"...
18. ...Lucas Lee wears a belt buckle with two Xs on it, and has a tattoo of the number 2 on his neck...
19. ...Todd Ingram wears a shirt with the number 3 on it...
20. ...The bar where Scott fights Roxy Richter is called "4"...
21. ...The numbers on the Katyanagi Twins' amp are written in Japanese kanji. The number they turn it up to is 11, which is not only a reference to This Is Spinal Tap, but is also 5 and 6 added together...
22. ...And the seventh and final ex is Gideon Gordon Graves. G is the seventh letter of the alphabet.
