Non-Prog Artists Doing Slightly Prog Things?

For sure, especially in the 70's when you had non prog people like The Allman Brothers, Steely Dan, Elton John, The Doobie Brothers, Loggins and Messina, Chicago(admittedly much of their early stuff was pretty progressive but they aren't often thought of as prog), Sweet, Led Zeppelin, Billy Joel and several others dabble in prog to one degree or another. Even the Rolling Stones had "moonlight mile" which I consider pretty prog. Come to think of it so was "Can't you hear me knocking" which has a second half that reminds me a bit of Traffic.

If you look at later music there were new wave, gothic and alternative bands who had bits of prog in their music whether it was accidental or on purpose.