The strangest use of pre-written modules I've used is also from my Unua campaign, as a small adjunct to the Infinite Staircase. The party found themselves in a pair of expansive gardens (each seemingly contained inside the other, and part of the Nine Hells to boot), that happened to be the two Alice modules by Gary Gygax (Dungeonland and Land Beyond the Magic Mirror), but with all the explicit Alice references taken out.
It all seemed to be have something to do with one of the party NPCs, a tiefling (infernal-blooded) called Slizar, and the two sides of her inheritance (represented by a set of remains in each garden, human and pit fiend).
It developed out of the Jabberwocky adventure from TSR Jam, but since that wasn't actually very good, I expanded it greatly by going back to Gygax, just taking out most of the combat (again), and emphasising the weirdness. There were no giant sentient eggs and the like, but there were roughly equivalent characters wandering about, and a complex set of rules that the characters had to work out to break the trap and release Slizar (and, in the meantime, have the opportunity to discover a very nasty trap a devil had set for them down the line).
Had some awkward bits, but in general it worked very nicely.