A song you didn't realize was a cover?

I got this album from eMusic because one of the tracks was the Free Daily Download. A few months later my then-girlfriend's sister was in the car with us when this song came on. She started singing along. My gf and I were shocked because the sister only ever listed to Top 40 stuff and we had no music in common.

Fast forward a few months when I was in a restaurant and I realized I recognized the lyrics to the song they were playing as Cry Me a River but it was clearly a very different version. I used my phone to ID it and found out it was Justin Timberlake and suddenly the sister knowing the song made sense.

I've always lived in a musical bubble but this is one of the best examples.