This didn't happen to me but I was there when it happened, some 30ish years ago.
My aunt, who was a serious hippy at the time, was going to make a red velvet birthday cake for her daughter (my cousin). She did not want to use red food coloring because she thought it would cause cancer (this was when red M&Ms had been banned for the same reason). Instead, she used beet juice, thinking it would make the cake red just like food coloring.
The color of the cake after it was baked was an awful brown and the taste was terrible! It was so bad that she gave in and made another cake, this time with red food coloring.
So, if any of you are considering substituting food coloring with something a bit more natural, you might want to stay away from vegetables.
lol... that's so funny, a friend and I were discussing beet cake a few days ago.. lol!!