I feel for you! This has almost happened to me a few times, but I noticed at the last second. After reading this thread I am thankful that I didn't get caught with a $1K+ bill for something way overpriced. The check-out process is so long and tedious that I found myself just thinking "click, click, type, click, click, type, type, click, click, click" and be done with it. The only thing I was screwed with once was a 5-year registration on a quick-decision purchase. I was going for speed and didn't notice the length of time - my fault? Of course, but the GD system makes those mistakes more likely than other registrars.
I am glad that you posted this thread, it will definitely educate other forum members (and clearly some guests) as to some of the problems with the GoDaddy check-out.