Just thinking about parts of an album (or at least of some greater work) that are a giant mystery to you after playing.
Jesus & Mary Chain - Honey's Dead: everything except for "Reverence" (an obstacle fatal to an attempted discography run in days past)
The Who - Sell Out: everything after "I Can't Reach You" (I do remember it being as great as everyone says, so maybe the fault is mine.)
Mendelssohn - any of the Songs Without Words after the very first one
Fletcher Henderson - any given compilation (Ken Burns for me): These are great when they're on but I can actually remember maybe five or six of the tracks, if that.
Telemann - entire output