Sometimes I'll hear fragments of a song and I'll be able to place the name of the band and what type of song it is (full vs instrumental, medley, transcription, etc. for example) but not the song name. And I'll get to a point where I consciously realize that the song is going through my head. Then that part of the song will start looping in my mind and even though it just played through my head I won't be able to fully recall exactly how I got to that point in the music, and it drives me nuts. (A TSO song as of late, pretty sure it starts in C and some point it modulates into F major and then ends in F minor, and since I can't recall the beginning, I'll have to go through up to an hour's worth of music to figure it out.)
Or I'll have a rough idea of how it progresses and I'll let it loop and then suddenly I'm back to a part where I've already looped and I'm like, "Wasn't there supposed to be something else here that led me to this?" (Third movement of the Pines of Rome right here.)