During an exam, it is important to be careful! I am always surprised by students who mistakenly fill out their answer sheet and end up "off-numbered" in some way. I always worry about students who spend time on a question, decide it is too hard to answer, skip it to go on to another question, and then make a mistake in filling in their next answer.
That said, I also question the wisdom of spending time on a question and not answering it with the thought that he or she will come back to that one later. It is very likely that the student will not have time to go back to all of the skipped questions. It is a better strategy, if you spend time on a question, to put an answer on the answer sheet and then mark in the test booklet that that question is one to go back to if time allows. That way, all of the questions have a "best chance" answer (and there is no penalty for guessing). If the student spends time on question and has to "start over" in his thinking about the question when he returns to it later, that initial time is time wasted.