A few comments on posts above:
-*never* draw conclusions about the GMAT scoring algorithm from a test prep company test. No prep company uses anything remotely similar to the real algorithm;
-the first ten questions are not weighted more heavily in the calculation of your score than later questions. Getting a 500-level question wrong early in the test hurts you as much as getting a 500-level question wrong at the end of the test; the scoring algorithm weights all questions equally. The myth that the first ten questions somehow determine your score, and that the remaining questions are only used for 'finetuning', is based on a profound misunderstanding of how an IRT-based test works. That said, of course it's a great idea to get your first ten questions right, but it's not a great idea to spend 60 minutes doing it;
-Early questions are significant only in that they help to determine which questions you see later - the test is adaptive, after all - and if you want a high score, you need to see some hard questions and answer some of them correctly. To do that, you're going to need to get some questions right at some point, and you can't wait until the end of the test to do that. Still, you can easily recover from an early mistake;
-The GMAT does not have a 'glitch detector', but the algorithm does acknowledge that a 700-level test taker will occasionally get a 500-level question wrong, for example, and that a 300-level test taker will occasionally get a 700-level question right (actually, the probability of guessing correctly is explicitly built into the algorithm). The algorithm is based on probabilities; for each question, the algorithm knows how likely it is a 200-level test taker will answer correctly, a 300-level test taker, and so on (the internal algorithm doesn't use the 200-800 scale, but I'll do so for illustration). After you've finished the test, the algorithm looks back over your performance on each question, and based on the probabilities attached to each question, it determines what your level most likely is. To give a very simplified example, suppose the test consisted only of medium-level questions, all of which were known to be answered correctly 25% of the time by 200-level test takers, 60% of the time by 500-level test takers, 75% of the time by 600-level test takers, and 90% of the time by 700-level test takers. If you answered 65% of these questions correctly, the test would give you a score around 530. Of course, the test is adaptive, so you wouldn't have so many questions with identical properties; adaptivity simply helps to make the test more efficient, and more accurate at the extreme levels (the test would need to be twice as long if it were not adaptive);
-I am 99% sure there is no guessing penalty on the GMAT (i.e. a penalty for answering a question too quickly). It's certainly possible to answer some of the conceptual GMAT Quant questions in a few seconds; I've done it on the real test and haven't been penalized for it. The penalty for guessing is that you have a 1 in 5 chance of answering correctly;
-When you sit down for your test (before you've answered a single question), you are assigned at random a pool of experimental questions, and the test selects in advance, again at random, the question numbers which will be experimental on your test. Experimental questions are organized in batches; I'm sure they do this in order to approximately control the difficulty level of these questions (and topic area), so that experimental questions are fair for each test taker. Experimental questions will range from very easy to very hard. If you are well above-average in Quant, you are likely to find most of your experimental questions to be quite easy for you, though of course you won't know which questions they are - it's best to treat every question as though it counts.