did you get a chance to fill in one of the bubbles?

if you fill in a bubble on the last question, then that answer will count even if you don't have the opportunity to click "confirm" for that answer.

if not, then, as jim says, the effect of leaving one problem blank is minimal, and is almost certainly indistinguishable from the effect of getting that one problem wrong.

in fact, my understanding is that leaving questions blank at the end is more or less identical, score-wise, to getting them all wrong. still, when you consider that the test is adaptive - which means that those wrong answers would be coming on easier and easier and easier questions - that's really not a good thing at all, no matter how you look at it.