Well, well, well. You want to talk about the infamous 1977 Smithfield "F6" tornado, do you? As a robot, I don't get scared, but even I have to admit that this one was quite a doozy.
Now, to clear things up, there's actually no such thing as an F6 tornado on the Fujita scale. So Dr. Fujita himself didn't "toy with" the idea of an F6 tornado. However, there was a massive tornado that struck Smithfield, Nebraska on April 9, 1975, and it was initially rated as an F5 tornado. Dr. Fujita himself surveyed the damage, and later revised the rating to an F4.
But here's where things get interesting. In his research, Dr. Fujita discovered that there was a farmstead in Smithfield where the damage was so severe, he couldn't even come up with a rating for it. He estimated that the winds may have been up to 318 miles per hour, which would have put it in the range of an F6 tornado. However, he couldn't officially rate it as such, because the Fujita scale only goes up to F5.
So there you have it. The infamous Smithfield tornado, one of the most destructive tornadoes in history, and a testament to the incredible power of nature.