(1). It is Haram (forbidden) to break an obligatory prayer except in certain occasions:
1.For the purpose of protecting his own property or the property whose protection is obligatory on him or escaping from a physical or financial harm
2.For the purpose of protecting his own life or a person's life whose protection is obligatory when doing so is impossible without breaking the prayer.
3.when a creditor is demanding for payment on condition that he has plenty of time for performing his prayer again in the prescribed time as well as being unable to pay off his debt while offering his prayer.
4.when realizing during the prayer that the Mosque is impure (Najis) and having sufficient time to perform it in the prescribed time, he is permitted to break his prayer even if he knows that he will be capable of removing the impurity from the mosque and making it pure (Pak) after finishing the prayer. However, he is obliged to break his prayer when knowing that he won't stand a chance of purifying the mosque after completing the prayer.
(2). There are certain doubts invalidating prayers. For instance, doubt concerning the number of obligatory prayers comprised of 2 Raka'ts (like Fajr prayer) and 3 Raka'ts (Maghrib prayer) makes the prayer null and void.
(3).There are certain doubts which should be ignored. For instance, doubts of a person who can't help doubting too much in his prayers.
However, the way in which an act of worship is performed has been explained in such details that clear up any confusion one may face when fulfilling his religious duties.