As in you rarely see them get mentioned by the fandom (positive or negative) and when you do remember the song it's kind of "oh yea that exists!" They aren't even necessarily the bad title tracks, something about them just didn't click long-term and kind of just fade out of the fandom's memorable songs
For me...
Twice - Heart Shaker
I rarely ever see onces mention this song and feel it's definitely an afterthought when it comes to Twice's discography. I also considered The Best Thing I Ever Did but maybe it's just the spaces I'm in but I actually see even that song being mentioned more. I actually like this song (used to have it in my top 10 Twice TTs) but I think the problem of Heart Shaker is that it felt a lot like rehash of the songs Twice had released leading up to it without the spark to make it memorable years or even months later. I remember the feeling of it being released kind of left me with an "oh... ok" feeling despite enjoying it. Technically a good song but doesn't have enough good or bad qualities to make it something people think about it
I've actually seen a carat surprised to find this song exists after stanning them for a while.... lmao. Like Heart Shaker, I actually enjoy this song when it comes through but it definitely lacks something. I think Boomx2 was especially hurt because it was the song following Aju Nice which is easily one of Seventeen's most memorable songs + already being a song that would fall a bit flat. Despite actually being a Boom Boom enjoyer, I do have to regularly remind myself that there was a comeback between Aju Nice and DWC (which is funny because the album that came with Boomx2 is one of my favorites of Seventeen). I think a close runner up would be Fear which was outshined by it's bsides, specifically Hit which I've seen people think was the title track
I really like 
Onces with taste remember!!!!
That's 
