A few years back I worked with a guy who bought a used Gravely. It had many attachments including a brush hog, rotary plow, and event he sulky seat.
The guy he bought it from just wanted it gone, and had many stories why: On one of it's first uses, he was mowing down a berry thicket, rode up on a little ridge, and the cutter head flipped over. Right before it flipped, it grabbed a couple strands of barbed wire that it proceeded to flog this guy with as he tried to get it shut down.
After recovering from that, he was using the rotary plow when it grabbed and threw a rock, breaking his arm at the shoulder.
After all this, he's just trying to use the machine, riding along on the sulky seat, making a turn, when the outside wheel climbed up onto a rock. This of course pushed the handlebar down tight into the seat, which wouldn't be a problem if certain parts of his anatomy hadn't been in the way. From what i understand, he sold the machine very cheap.