In an autosomal Recessive (AR) disorder, one parent is normal and the other is carrier and the child is also affected. What is the reason?

If one parent is normal for an autosomal recessive disorder and the other is carrier - No child should be affected. But there is exception to this rule, i.e. uniparental disomy (UPD), when a person receives 2 copies of a chromosome, from one parent and no copies from the other parent. Ex: In Prader-Willi syndrome- maternal UPD of chromosome 15 (missing the paternal chromosome 15). In Angelman syndrome- paternal UPD of chromosome 15 (missing the maternal chromosome 15).