This came out a year ago, but my friend called it to my attention again a few hours ago. Some paleontologists now believe that Torosaurus was actually an older specimen of Triceratops.
Makes sense on some levels. The skeletons are almost identical, except that Torosaurus had a longer frill. No juvenile Torosaurus specimens have ever been found. And Tricertops frills tend to become longer and thinner as they get older.
But those paleontologists are a feisty bunch. I'm sure some of my dinosaur-loving friends will be passionate about this one way or another.