Some I learn, some I don't. Some come to me after hearing them for years and never giving them a second thought..."Lay Down Your Weary Tune" comes to mind as one of those.
The other night I went to sleep with a tape playing in the VCR of a movie. I had taped other things following the movie but I had no idea what. Turns out that there was a televised episode of Prairie Home Companion (maybe 10 years old?) at the end of the tape and I woke up to Emmylou singing an Allen Reynolds song, very gospelly/bluegrass, called "Someday My Ship Will Sail." Its very short, but I can tell you its already hit the category you're talking about. It just grabbed me and won't let go. BTW, her mando player was a young and unknown Vince Gill.