Ok, I asked for a bit more detail about it. Still dunno how long we've known about it but I did find out more about what it does. Well as stated above, acetaminophen screws your liver. The grapefruit apparently has something that temporarily impairs your bodies ability to filter/pee it out, so it just sticks around in your liver longer, thus allowing it to do more damage.
What I said earlier about the stomach was a misconception, the stomach thing I was thinking of was that ibuprofen is bad for your stomach lining(pretty sure at least).