Pat Boone's 1997 album, In A Metal Mood: No More Mr. Nice Guy was completely composed of covers of heavy metal songs, as opposed to his usual fare of R&B, country, and gospel (though they were still done in his milquetoast Big Band/Lounge-style, of course). The CD cover featured the normally clean-cut Boone in leather and chains, an outfit he would also wear at the 1997 American Music Awards. He looked ridiculous, but it didn't do him any favors with the Moral Guardians that make up his primary demographic, who thought he was being serious.
This review proves that, amazingly, even some critics didn't get that it was a joke.
The Colors Album by Between The Buried and Me (Can't say anything about the other albums) has this in each song. For example, Ants of The Sky has polka, Pink Floyd-esque guitar solo, thrashing a-la Metaliica and Megadeth, and intense speed-metal screaming, melodic sections that would make Joe Satriani and Dream Theater proud, and thudding doom-metal sections. In a single song. The thing is that the styles switch nearly immediately, and doesn't sound bad.
Neil Young put out three such albums in a row, much to the consternation of his label (Geffen famously tried to sue him for delivering "unrepresentative material"): First there was the heavily synth-filled Trans, then The Fifties rockabilly throwback of Everybody's Rockin', and finally the country album Old Ways.
The first eleven songs on the RemedyDrive album Magnify are all pretty standard rock (and the occasional bass solo). The last one, "Smile Upon Me", is acapella three-part harmony.
By 1993, R.E.M. had 2 massively successful albums with Out of Time (Shiny Happy People, Losing My Religion), and Automatic for the People (Everybody Hurts, Man on the Moon). Both albums, especially the latter, were relatively slow, emotional albums, with string and acoustic instruments everywhere. In 1994, however, they released Monster, with loud, grunge-y, distorted guitar on every single track.
Incidentally, those two prior albums fit the trope as well, as the band's major label contract was triggered by the success of the 1987 album Document (It's the End of the World As We Know It, The One I Love), which consisted almost entirely of songs that were, by the band's standards, real rockers. The interceding album, 1989's Green, seemed to be following that pattern, with just a few acoustic songs between upbeat rock songs like "Stand," "Orange Crush," and "Pop Song '89."
Keane's first 2 albums were straight piano rock, very similar to Coldplay or Ben Folds. Their third album, Perfect Symmetry, had a very 80's inspired sound featuring heavy synths, prominent basslines, a wide array of instruments, and, for the first time since well before the release of Hopes and Fears, guitars. Their next album, Strangeland, was straight-up piano rock in a similar style to Hopes and Fears.
"Golden Brown", a sincere, harpsichord-led baroque pop single by punk band The Stranglers. It is their most acclaimed, popular and best selling song.
This goes for their entire career from that moment on, as they shifted to moody baroque pop, and later to AOR.
"Revolution 9" by The Beatles isn't like anything else out there.
Minor example: A Day in the Life is a soft, sad-esque song about a guy who reads in the newspaper the story of an unlucky man, the war, a car crash, a suicide, etc. By the middle of the song, it starts an upbeat ballad about going late for work.
Hardcore Punk band The Bronx, after releasing three self-titled albums, released an album of mariachi music under the name "Mariachi El Bronx" in 2009, followed by another in 2011. Both albums are critically acclaimed.
Outkast did this twice in a row. 2003's Speakerboxxx/The Love Below was a double album with each disc being a solo effort from one half of the duo: Big Boi's Speakerboxxx a conventional hip-hop album and Andre3000's The Love Below being more experimental. In 2006 they released Idlewild, the soundtrack to their 20's-era musical, meaning most songs were more of a jazz style, plus there were only a couple songs that Big Boi and Andre performed together. To recap, Outkast has not released a standard album since 2000's Stankonia.
Bob Dylan's been known to do this from time to time and has had many different genres but Empire Burlesque stands out being a uncharacteristic (though not for The Eighties) heavily produced synth pop affair (with one song even leaning slightly towards disco) and then he does it again in the same album by having it end jarringly with the quiet "Dark Eyes" a simply structured track that features only Dylan, a guitar and a harmonica without any studio embellishment whatsoever.
JOJO did this in "The High Road" in this song Coming For You which is pop rock instead of loungy R&B of the album.
Da Yoopers' 1992 album Yoopy Do Wah, the last full album to feature original guitarist Joe Potila, was also the only album after their first not to include comedy skits between the songs. It also included "When One Love Dies", their first serious song since the Early-Installment Weirdness of their debut.
Alan Jackson did this twice in 2006: first with a gospel album called Precious Memories, then a few months later with Like Red on a Rose, a smooth, ballad-heavy AC album that was a radical departure from his neotraditionalist country sound. The latter was also the only album of his career which Keith Stegall did not produce (bluegrass singer Alison Krauss produced).
The Beastie Boys' Aglio E Olio (a Hardcore Punk EP) and The Mix-Up (an album of instrumental soul-jazz). Neither style was entirely new to the bandnote They actually started out as a hardcore punk band and occasionally made nods to those roots, and funk and jazz influenced instrumentals were prevalent on Check Your Head and Ill Communication, but they were still pretty unusual outings for a rap group.
George Strait's 2009 album Twang is a double example: it had his first songwriting credits since his 1981 debut album, and it included a mariachi cover. 2011's Here for a Good Time includes more co-writer's credits, one being the very atypically dreary and downbeat "Drinkin' Man". The latter was his lowest peaking single to that point.
Kenny Chesney released Be as You Are: Songs from an Old Blue Chair in 2005. It was a laid-back, acoustic album based around the song "Old Blue Chair" from his album When the Sun Goes Down two years prior. Be as You Are intentionally did not produce any singles. Despite being a side project, it went to #1 on both Top Country Albums and the Billboard 200, and was certified platinum. It actually ended up being a subversion of this trope, as his "default" style shifted to the more acoustic, introspective bent that Be as You Are had.
Japanese pop star Ayumi Hamasaki does this on most of her recent albums. 2005's "(miss)understood" started off with an upbeat gospel anthem only to jump to electropop 4 minutes later - and then to rock...and then dramatic over-the-top ballads. She somehow manages to put songs inspired by symphonic metal, classic rock, musical showstoppers, pop ballads and electropop in very, very close proximity on 2012's "Party Queen"
Florence + the Machine's "Kiss with a Fist" is more rock than all the other songs on the album Lungs, due to being based off an earlier indie rock song called "Happy Fist".
In the documentary Long Way to the Top: Stories of Australian Rock & Roll, Angus Young of AC/DC recalls that, for some reason, the band decided to play the theme from Zorba The Greek at one show.
EBM pioneers Front Line Assembly pulled this at least twice, first with the Industrial Metal album Millenium, then the IDM/D&B album Flavour of the Weak, and to a lesser extent the dubstep-influenced AirMech soundtrack.