The standard example of this is the question Have you stopped beating your wife? Whether the person asked answers yes or no, he will admit to having beaten his wife at some time in the past. Thus, that fact is presupposed by the question, and if it has not been agreed upon by the speakers before, the question is improper, and the fallacy of many questions has been committed.
Note that the fallacy is all in context: the fact that a question presupposes something does not in itself make the question fallacious. Only when some of these presuppositions are not necessarily agreed to by the person who is asked the question, do they become fallacious.