Considering the steps that the elders take to be sure that a person is qualified for baptism, it would be extremely rare for anyone’s baptism to be invalid.
لِذَا مِنَ ٱلنَّادِرِ جِدًّا أَنْ تُعْتَبَرَ مَعْمُودِيَّتُهُ غَيْرَ صَحِيحَةٍ فِي مَا بَعْدُ .‏