In the first episode of the anime, Mugi is making a big deal out of mixing her fries with those of the other two girls. In Japanese culture, sharing your food with the group is a sign that you accept the group as your own, so Mugi was both being formal and very sweet in one go.
The train station seen most in the series, which is the one Mugi uses for commuting, is the Shugakuin Station on the Eizan Electric Railway in Kyoto.
The high school is based on an elementary school in Toyosato in the Shiga prefecture. The building was supposed to be torn down, but after protests from the local community it got renovated and opened for the public, and it now serves as a museum. It's needless to say that the building is now a very popular destination for fans of the series.
The Max Burger fast food restaurant frequented by the girls is based on a real-life McDonald's.
In episode 5 of the first season, Sawako plays her super flashy solo. However, the way she moves her arm while "speed picking" is something more akin to new guitar players. Faster playing is done using mostly the fingers or the wrist, Sawako's using her arm almost exclusively.
However, some fast guitar players (shredders) do play fast using the elbow, but even then, the motions are much smaller than Sawako's.
The place where the girls go to feed the Japanese monkeys, as seen in in episode 4 of the second season, is the Iwatayama Monkey Park in Kyoto, situated on top of Mt. Arashiyama. It is pretty much as depicted, including the splendid view of the city.
The statue seen at the very end of episode 7 of the second season is based on a statue of◊ William S. Clark, the founder of Sapporo Agricultural College (now Hokkaido University), which implies that Sokabe-sempai is studying in Sapporo. Also, the full version of "Cagayake!GIRLS" contains the words "Girls, be ambitious", which is a reference to Clark's famous parting words to his students ("Boys, be ambitious!").
The festival the girls and Sawako attend in episode 12 of the second season is based on the Fuji Rock Festival, which is held annually in the Naeba Ski Resort, in the Niigata Prefecture of Japan.
The girls are named after former band members of the Japanese New Wave band P-Model. Yui is named after Susumu Hirasawa, Ritsu after Sadatoshi Tainaka, Mio after Katsuhiko Akiyama, Mugi after Hikaru Kotobuki, and Azusa after Teruo Nakano.
Their teacher, Sawako is named after Sawao Yamanaka of The Pillows.
Ascended Fanon: Fan speculation (NSFW site, SFW article) led to the girl sitting next to Yui in class being referred to as Himeko Tachibana. The name itself came from an official list of classmembers, but no specific character was attached to the name, and while the girl in question did have a line, she was only credited as "Classmate". The character profiles in the Season 2 DVDs confirm that she's Himeko.
Bonus Episode: Episode 14 was included in the first season's final Blu-ray volume. It followed the girls New Year's Eve, but with a new twist not in the manga; they performed at a local live show. After all that was done, the episode shifted back to following the manga at the end.
Cash Cow Franchise: April, 2010. The pre-orders for the yet-to-be-released DVDs of K-On second season occupied ten of the Top 13 most bought movies in amazon.com Japan. And this is only after the first episode of season 2 has aired, folks. The momentum of success and die-hard fandom set by Season 1 in 2009 does not look like it will slow down any time soon, with twenty six episodes in this season altogether.
A movie came out in late 2011. Guess what the general reaction from the fans was...
It's getting worse and worse. K-On model plane, K-On video game...
There're also character cookies (quite horrifying when you realize you're basically eating the girls in cookie form), as well as Giitah cookies (yes, you CAN eat Yui's guitar), and other assorted snack foods. Now with pictures.◊
Cross-Dressing Voices : Satoshi, Ritsu's younger brother and the male character who appear the most, is voiced by Mika Itou who is actually female.
Defictionalization : A cassette with the tracklist the band put together in season 2 episode 23 was released three weeks after the series ended its broadcast run.
A third dub was made by Sentai Filmworks, but it was for the second season. It doesn't really count as this trope due to the entire voice cast reprising their roles from the Bang! Zoom! dub.
Fake Brit: Averted in the English dub of The Movie, at least in regards to the sushi shop owner, who is voiced by Martin Billany (aka LittleKuriboh).
Follow Up Failure: The anime's roaring popularity couldn't help the College and High School spinoffs from being canceled after one volume each.
I Knew It!: Because the final bonus episode had the girls getting their passport pictures taken, fans predicted that the film would be about the girls going on a trip to another country. ....guess what!
The Merch: From shirts to ties to guitar covers to Image CDs to replica guitars◊ and hilariously overpriced rice. It safe to assume it won't be going away for a while.
Ever since working together in this anime as main characters, the five Japanese voice actresses have regularly worked together in rotation in many other series.
The Japanese voice actresses of all 5 main girls have appeared in Hyouka.
Seitokai Yakuin Domo also sees another pair of friends voiced by Yoko Hikasa (Shino) and Satomi Sato (Aria). Both characters inherit hair colours from Mio and Ritsu in K-On! (black and brown), but their breast sizes reversed.
Also, all their voice actresses also worked in the Queen's Blade franchise, albeit not everyone at the same time.
Sawako Yamanaka (Karen Strassman) and Nodoka Manabe (Laura Bailey) teamed up in Sakura Wars as two of the six combat pilots in the New York-based Star Division, while Mio Akiyama (Cristina Vee) fittingly worked behind the scenes as one of the team's support personnel.
Sleeper Hit : The original Yonkoma was virtually unknown before being turned into an anime.
Star-Making Role: The main characters of K-On! were this to all of their Japanese VAs, and some of their English VAs as well.
Yui to Aki Toyosaki. Though Toyosaki seems to have avoided the typecasting that typically comes with successful roles. Presently, she has a tendency of taking perverted and sultry seductresses who are nowhere as sweet and innocent as Yui is. Averted with Yui's English VA, Stephanie Sheh, who had already made her mark as Orihime and Mikuru.
Ritsu to Satomi Sato and Cassandra Lee. Interestingly, although Ritsu is one of Satou's most famous roles, a lot of the characters she voiced before and since are quiet and demure, directly against Ritsu's type. Most notable of these roles is Eru Chitanda from Hyouka.
Mio to Yoko Hikasa. The creepiest part is the resulting roles seem to allhavethathair. Her role as Mio also paved the way for Hikasa's musical ambitions, providing themes of many other anime series since.
Mugi to Minako Kotobuki and Shelby Lindley. Kotobuki, in particular, has recently become the go-to-girl for Ojou characters, though these characters tend to be more domineering, bitchy, and substantially less nice and innocent than Mugi.
Azusa to Ayana Taketatsu and Christine Marie Cabanos. It is worth noting that Azusa was Taketatsu's first major role, debuting in the year of K-On!'s premiere and having voiced only in one OVA episode earlier.
A cassette recorder is used to record the band's complete song list, of all things.
Apparently, Ritsu carries around Polaroids of pan pastries at all times just to make a bad pun. The show narrowly missed the smartphone revolution, which would've allowed her to access thousands of weird, random pictures with the tap of a screen. Google Maps would've stopped them from getting lost in Kyoto, too.
Urban Legend of Zelda: In 2011, rumors circulated that Disney Channel was going to air the English dub of K-On!, but with several edits such as removing all swears, eliminating Mugi's Shipping Goggles, toning down Megumi's stalker behavior towards Mio, and replacing the cake and tea with McDonald's and Coca-Cola. Of course, this never happened, though K-On! has actually aired on Disney Channel's Japanese feed in edited form (exactly what was edited is unknown).