Wow, that's a tough way to learn this lesson, shivendra, but for everyone else please keep this in mind:
On your GMAT you will see *several* (it's definitely more than 10) unscored, experimental problems that GMAC is running through the pool to gather data. There's a decent likelihood that some of those problems are flawed, including:
-A question is culturally biased (the right answer is right and the wrong answers are wrong, but there's something in the subject matter that favors people from a particular region or background)
-A question is ambiguously worded (in trying to "hide" the key to unlocking the problem, the author was unable to include enough information for it to reasonably or concretely be solved)
-A question is too labor-intensive (it's not "wrong" but it takes too much time to be fair within the time limits of the exam)
-A question has a second correct or defensible answer choice
-An "insufficient" DS statement is actually sufficient if one draws on a field of study (say trig or calculus) that the authors didn't anticipate
-A question is formatted poorly (like what you saw)
-A question is missing key information (like "not drawn to scale" on geometry for example)
-Etc.
Like GMAC said, those questions won't count until they've been statistically confirmed and "graduated" into the live, scored pool. But like you saw, it can be distracting or unnerving to feel like you've found a flaw or are facing an unfair question. So just know in advance that there is a fair likelihood that you'll see an unfair question. And if you do see an unfair question, there's an incredibly low likelihood that that item itself will affect your score. So just do your best and if you're really distracted, tell yourself "it's experimental." I've had multiple students come back from their tests and claim that there was an unsolvable Problem Solving question or a verbal question with the same answer repeated twice or something like that. And there's a good chance that was the "fog of war" talking...their minds were spinning and they were exhausted and they just blew it. But maybe they did see that, and if so you have to be able to tell yourself "it probably doesn't count, so make a reasonable decision and move on."
This is also why it's a terrible idea to try to read into your performance by the difficulty level of the problem in front of you. If you see a dead-easy problem at #15 it may, indeed, be dead easy and you might as well go home because you're performing/scoring so poorly. BUT there's also a fair likelihood that it's an easy experimental problem and they're gathering data on how well a genius like you handles such a problem (if more than a few 750 scorers get a 300-level problem wrong, it's probably flawed and GMAC needs to know that). Or what makes it hard is that most people don't even see the little trap lurking there. Either way, if you see a way-too-easy question or a way-too-hard problem or a potentially-flawed problem, just tell yourself that it could be experimental and do your time-efficient best just in case it does count. And then be ready for the next question.