Dirty Deeds done with sheep
Dirty Deeds and they're done with sheep

The story:

Well, it could have been worse...a lot worse. See, my brother is kind of a pervert and he enjoys changing the words of songs. He is the one I heard sing that version and I never realized that it was wrong because, you know...it's AC/DC. My brother and I were in the living room singing along to the song and we both sang the sheep version, then right before the song ended, I stopped and actually listened carefully to the words. I realized my mistake and the realization was confermed by the announcement of the song title and artist. I was so shocked that I blurted "It's not dirty deeds done with sheep". My brother looked at me in surprise for a moment, then started laughing hysterically.
I was really embarrassed. It wasn't so much that I had been singing the wrong words, but that he had fooled me (indeed, since then I have sang the wrong words a few times just to get a laugh). I was thankful nobody else was in the room!