There are certain things or actions which invalidate a prayer:
1. Hadath whether the greater or lesser, and whether intentionally or by mistake.
2. Turning away from qibla.
3. Anything which disturbs the prayer (i.e. anything which gives the impression that the person is not praying), like remaining silent for a considerable time, clapping the hands or jumping or indulging in actions other than the prayer.
4. Speaking intentionally even a single letter (other than prayer or Qur'an or dhikr of Allah). But replying to Salaam al-Shar’i ('Salaamun Alaykum' or 'Assalaamu Alaykum') is allowed in the very same wordings.
5. Laughing intentionally. Smiling does no harm to the prayer.
6. Weeping intentionally for worldly affairs. But weeping in love or fear of Allah or because of remembering the suffering of Imam Husayn (A.S.) has no harm.
7. Eating or drinking however little may be, whether intentionally or by mistake.
8. Folding the hands intentionally as some non-Shi’ah sects do in the prayer.
9. Saying ‘Amin' intentionally (with exception in certain cases) after Surah al-Hamd.
10. If any of the conditions of prayer is violated. For instance, he realizes during prayer that his clothes are ghasbi.
11. Doubt in first two rak’ats of a 4 rak’at prayer; or anywhere in a 2 rak’at or 3 rak’at prayer.
12. Adding or leaving out any rukn of prayer.