"Can't You Hear Me Knocking?" by The Stones
A hard rocker for its first third and then a chill Latin groove for its duration.
Over the Hills and Far Away
Led Zeppelin
The Winner Takes It All - ABBA
In the sense that it starts out all magnanimous and contrite but quickly devolves into jealousy and spite.
Stereolab - Refractions in the Plastic Pulse
https://www.youtube.com/watch?v=fnfjkDkHLaw
Stereolab have a number of multi-part songs that could fit here. Sadly, a lot of their material is not on YouTube. Seek out, for example, Good Is Me, which transitions between a straightforward rock jam and a cosmic country interlude before finally ending with the most righteous prog rock outro I've personally ever heard. Another great left field track of theirs is called Long Life Love, you've never heard any other thing like this one, I can guarantee you.
shareBohemian Rhapsody.
shareAnother band that I think can loosely be fitted into this conversation is of Montreal. They don't have a lot of songs that take unexpected left turns (though they do have some), but what they do have is an unusually developed talent of transitioning from one track into another, so that they seem to flow effortlessly together. To hear the record where I think they do this most effectively, check out "Skeletal Lamping." You can also check out their record "Paralytic Stalks" to hear probably some of their (his, Kevin Barnes) most unconventional song writing.
shareTortoise subverts structure quite lovelily! (I think I might've just coined a term...)
Examples:
https://www.youtube.com/watch?v=WaXSHE54XLg
https://www.youtube.com/watch?v=lYatqE7OA1Q
'Always loved the turns this Sea & Cake tune takes:
https://www.youtube.com/watch?v=rr62vsKD3Cg
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