The Camper Van Beethoven Albums Poll

"Five Sticks" uses a different (drum-free) mix of "Ambiguity Song" played backwards with overdubbed (forwards) acoustic guitar and tambourine.
"Circles" uses the chorus of "Oh No!" ("Oh no, here it comes again, that funny feeling") played backwards.
"Border Ska" (instrumental) and "Heart" (which has vocals) are both ska songs with very similar guitar melodies, if that's what you're thinking of.
The beginning of "Stairway to Heavan" is a somber version of "Processional."

Ernest comes through! Another one that comes to mind is the they recycled some of the background vocals from “Peace and Love” into “Eye of Fatima Part II” (“Cowboys on acid”, “No-one ever conquered Wyoming”).

It’s actually pretty astonishing how productive they were during their few years together the first time. Five LPs, an EP, enough outtakes and B-sides to make a whole other LP, not to mention 2 Monks of Doom records and 3 albums of Camper Van Chadbourne stuff, was all written & recorded between ‘84 and ‘89.

"We're All Wasted and We're Wasting All Your Time" features backwards stuff from "Devil Song" and "Take the Skinheads Bowling."

https://www.youtube.com/watch?v=E-kguFNwwko

It's on that "Camper Van Beethoven Is Dead. Long Live Camper Van Beethoven" comp, which is itself pretty confusing, since some of it is old and some of it is new, but it's not always clear which is which. For example, this track features parts of "Devil Song" backwards, but "Devil Song" didn't show up until "Our Beloved Revolutionary Sweetheart." But this "Wasted" mishmash also appears on the reissue of "Telephone Free Landslide Victory." So who knows.