I knew some time ago that I wouldn't be ready in Sept. -- the deadline just slipped my mind. And I didn't realize that the deadlines had been moved up. It's completely my fault. I'm just trying to determine the lesser of two evils.
I would show up and cancel after taking the test, although you could try playing off an absence as outside circumstances you couldn't control. But that would be lying
Honestly, if you had something seriously preventing you from taking it, and you could honestly explain why to the schools you are applying for, then that is great. But if it is only because you haven't studied yet, then I recommend you take the test and cancel it. Showing up and canceling right away seems pointless because you should at least see what it would be like. And a cancel will show up just as a cancel. I don't think there is any difference between canceling right away and canceling after the test. I may be incorrect on this.
Good luck though. If it had been me, and I had known by August that I wouldn't be able to take it, I would have postponed as soon as possible. I had some doubts going through that month and considered it, but in the end, after Labor Day passed, I knew I was locked in. Just had to keep studying and hope for the best.