Such uncertainty may result from questions about whether the Triune name of God was used by the person administering the baptism. In some cases, there are doubts about whether a church from which someone is converting baptizes in a valid manner. For example, the Catholic Church has said that the validity of baptisms in The Church of Jesus Christ of Latter-day Saints (which does not practice conditional baptism) and in some other communions is doubtful. It is an issue where an infant is a foundling, and it is not known whether the child had been baptized before he or she was abandoned. Another example of a case requiring conditional baptism is when an emergency baptism has been performed using impure water. Then, the validity of the baptism is in question. In that case, a conditional baptism is later performed by an ordinary minister of the sacrament with certainly valid matter.
In a typical baptism, the minister of the sacrament (in the Catholic Church usually a deacon or a priest, but sometimes, especially when the baptized is in imminent danger of death, a lay person or even a non-Christian) says
I baptize you in the name of the Father and of the Son and of the Holy Spirit
while pouring water upon the head of the one being baptized, or immersing him or her in water. In a conditional baptism, the minister of the sacrament says
If you are not yet baptized, I baptize you in the name of the Father and of the Son and of the Holy Spirit.
Only the living can be recipients of Sacraments. Thus, if it is uncertain whether the baptizand is dead (i.e., his soul has parted from the body; this is the case for the first few hours or so after death in the modern sense), the formula is "If you are alive, I baptize [...]". In severe cases of birth anomaly, the (practically, always emergency) baptism formula is "If you are a human being, I baptize [...]".
Likewise, if an emergency baptism has been performed over a part of the body other than the head (practically: during birth), or on a pregnant woman's womb (for the unborn child), the child is to be conditionally rebaptized (with the usual "if you are not baptized") even though the emergency baptisms should be performed in this way if necessary.