I hear the song in my head and start making what my brain indicates is the same sound that I’m hearing. Except it’s not even close. It’s like opening your mouth to say something in English and then hearing it come out in French. Except hearing someone speak French has never made me stop what I’m doing to make an “ew, what’s that smell?” face and say, “Good God, what was that?!”
My singing is so distractingly bad that it distracts me. I’m happily listening to a song, start singing, and then get all irritated because something is ruining my song: that would be me.