1) Discharging anything from the two natural orifices, 2) discharging anything that is deemed filthy and impure from the body, 3) entering into a state of unconsciousness as a result of sleep or anything else, 4) touching one's private parts (the sexual organ or the rectum) without a 'partition' [such as gloves], 5) eating camel meat, and 6) apostasy from Islam (may Allah save the Muslims from falling into it).
Important note: The act of washing the body of the deceased does not invalidate one's ablution. This is the opinion of
the majority of Muslim scholars, as there is no textual evidence from the Qur'an or the Sunnah to prove this. If the person who undertakes the washing accidentally touches the private parts of the deceased without a covering (on his hand), then he or she should perform ablution.
The person who undertakes the washing must not touch the private parts of the deceased without a covering.
Moreover, the act of touching a woman does not invalidate one's ablution, whether this is done with or without lust, as long as this does not result in a discharge. This is the preponderant opinion of scholars. The evidence brought forward in this regard is that the Prophet (peace be upon him) kissed some of his wives, then he prayed without performing ablution again.
As for Allah's saying in Surah AlNisa' and AlMa'idah, or you have been in contact with women (by sexual relations) simply means sexual intercourse. This is the most correct opinion of the Muslim scholars, including that of Ibn `Abbas (may Allah be pleased with him). Allah is the Grantor of Success.