Firstly: Persistent Hadath
Whoever is afflicted with a persistent hadath then performs ablution, then anything which exits his body after that does not nullify it. Ibn Taymiyyah has quoted agreement over this.
Secondly: Certainty in Purity and Doubt in Hadath
If a person performs ablution then doubts being in a state of hadath, then there is nothing upon him and he does not have to repeat his ablution. This is the position of the majority: Hanafis, Shafi`is, Hanbalis, and most jurists hold this view.
Thirdly: Certainty in Hadath and Doubt in Purity
If someone who has performed ablution nullifies it through hadath and he is certain of that, then he doubts whether he has performed ablution after that, then he is not considered in a state of purity. Consensus has been quoted over this by: al-Mawardi, Ibn Hazm, Ibn `Abd al-Barr, Ibn al-`Arabi, al-Nawawi, and Shams al-Deen Ibn Qudamah.