In my department (physics) we only have prelims (oral and written) and oral qualifying exams, but sometimes the qualifying exams are taken very late and act as de facto thesis defenses. You give a 1-hour presentation (including question time, so it's more like 30 minutes), and answer general physics questions for another hour. It's usually not a big deal.
My boss' first grad student did a TON of studying, but never got asked the general physics questions -- the professors only wanted to know more about his presentation. So for my boss' second grad student, he was specifically told NOT to do any general physics studying -- just make a good presentation. So he went into the room, gave his presentation, and they grilled him on basic physics questions for the next hour. It was terrible, he couldn't answer a thing, and had to derive basic solid state physics from first principles.
He left the room for the professors to deliberate, and the smartest, toughest theorist in the room says, "Well, of course he passes." They all agreed immediately, and one of them said, "Yeah, his presentation was great, but he really needs to learn his basic physics." The super smart theorist replied, "Agreed. But you know, I don't think I learned any basic physics until I had to teach it myself!" The other professors laughed and continued on.
Disclaimer -- this is one of the top physics departments in the world, and the student had published works in some of the top journals in the world as well. It is a fairly laid-back department, and it was clear that he knew his research. Other people HAVE failed their qualifying exams, and this guy felt absolutely horrible about it even though he passed. So I don't recommend that you do this yourselves. :)