In 2004, Christina Aguilera won the Best Female Pop Vocal Grammy for her massive hit Beautiful, , but the vocal you hear on the track is actually the demo cut. Aguilera wasn’t a star yet, and songwriter Linda Perry wasn’t sure she wanted to give the song to Aguilera, so Perry had her cut a demo to see how it would sound.
Since this vocal track was never intended to be used in what would eventually become a massive hit, the click track leakage from Aguilera’s headphones was likely ignored. In the final mix the engineers put a ton of compression on the track which helped eliminate most of the leakage.
Listen to the first verse from about 0:30 until the end of the verse and you can hear the click track at the end of each lyrical phrase. It’s particularly noticeable after the words insecure (0:44) and pain (0:47) and then again at 3:47. It sounds kind of cool until you know what it is, and then it becomes quite distracting.
So, on the plus side, what you’re hearing is a magical take that was done without the pressure of trying to be perfect, but on the minus side you’re hearing some technical imperfection that tends to slightly get in the way.