Today’s challenge (for which I have no answer):

Think of a pop/rock song with two vocal parts where at least 30% of listeners would disagree with the other 70% about which part is the melody/lead. Call/response not allowed, needs to be simultaneous harmony vocals. Octaves not allowed either (though I can’t think of a clear case where that’s an issue).

Baby’s in Black?

The Beatles – Baby’s in Black

According to Wikipedia:

That might be a bit self-serving since in my opinion, if I had to pick, I’d say that John (the low part) has the main melody, but it is more ambiguous than most songs, and there are definitely parts (e.g. the very short bridge) when the main line seems to shift to Paul. Also, if you were to just ask someone who had the range to sing either part to sing along with the song, they might actually choose Paul’s part since it cuts through better, or more likely, they’d be confused and shift between the two.

Crazy (or maybe not crazy, since you and I have played that song together a zillion times) that I also thought of Baby's in Black as my first answer, then immediately wrote it off (without listening again) because the first few lines of the high (Paul) part feel like a harmony, and on the "she thinks of him" verse, the high part really feels like a harmony (resolves to a third).

But what's really interesting in the opening verse is that without any crazy transition, on the line "oh, what can I do", the bottom part feels more like a harmony to me, and is a little harder to sing, and the top is doing the natural, more obvious resolution of the line. And throughout the song, the parts are mixed at almost equal volume; if anything the top line is mixed a little higher (which would argue for it being the melody).

So in retrospect I'm totally on board with this answer.

Unrelated: this song is also one of those songs that it's a little hard to decide what's a verse and what's a chorus, in the way that I think different people could each agree that there *is* a verse/chorus structure but disagree on what's what.

Still in Beatles territory: "Drive My Car" has a similar vibe for me. Actually both parts feel like a harmony. The chord progression is [D]→[G]; the high part sings G→G, and the low part sings C→B. Which is the melody?

The Beatles – Drive My Car

Not sure if this is the phenomenon you're looking for, but in Jeopardy by Greg Kihn:

Greg Kihn – Jeopardy

...there's a little thing right after "our love's in jeopardy, baby" where the following happens:

1. Vocals go "oooh, oo-ooh ooh" over a Dm chord, on the notes A-E-G-F.
2. Guitar goes E-G-B, where the E and G are aligned with the E and G in the vocals, but the B hits before the F.

When I listen to this, I hear it as the guitar stealing the lead from the vocals. So the A is lead vocal, the E-G is unison, and the F is a background vocal, while the B in guitar is a lead note. I'm guessing most people do not hear it this way, but I had to make a conscious effort to even acknowledge the F.

Wow, Ian reaches to the archives... how did you have a thorough analysis of "Jeopardy by Greg Kihn" chambered and ready to go?

I think I disagree with your assessment of that spot, though mostly I disagree with the fact that you made me listen to this song and watch the video.

I have a vague inclination that there are some male/female duets out there that will have sort of the same vibe as "Baby's in Black", a high part might have a few more thirds in it than we'd normally attribute to a melody, but the two-gender situation puts enough weight on both parts that we overlook that. My first thought was the chorus of "Don't You Want Me Baby":

The Human League – Don't You Want Me

...but a second listen makes me feel like the top part is really just a harmony, mixed lower than I thought, and I was looking for the chorus to sound more ambiguous because the male and female singers each sing one of the verses. So at the level of the whole song, it's less clear that the male is the lead... nonetheless I'm not giving myself credit for this answer because at any one spot, it seems to be pretty clear who the lead is. But I still think there are some male/female duets out there with this property.