The following categories of people fall victim of this mistake.
1. Those who wear tight trousers that describe the shape of their body parts or exposed it to such an extent that when they make ruku (bowing) or sujud (prostration) the cloth they are wearing on top (e.g. a shirt) will roll up thereby exposing their back or buttocks.
Exposing ones nakedness during sallah can nullify ones prayer
Sheikh Abdullah ibn Abdurrahman AI-Jibreen said: "Many among those who wear long loose fitting garment some of them usually wear trousers and wear upon it Jubbah (a long outer garments, open in front, with wide sleeves) whenever they bow (rukuh) their Jubbah would roll up and the trousers or panties pull away so that part of his back or buttocks will be exposed. This makes it possible for those behind such a person to see the nakedness of the person concerned. Exposing the awrah (nakedness) during sallah makes it invalid."
2. Someone who doesnt take her dressing seriously (especially the female) and she is not concerned about covering all of her awrah while standing in front of her Lord either as a result ignorance, laziness or lack of concern. All the scholars are agreed that the dress which a woman can put on in order for her Sallah to be accepted should be a wide and loose outer garment with a khimar covering her head (i.e. her garment should cover the whole of her body and head). If the garment is so wide that it can cover her head with the remaining of the cloth after covering her body with it, then it suffices. (Sahih Bukhari),
Ikrimah narrated that "If she can hide the whole of her body with a single piece of cloth, it suffices". Some women pray while part of their hair or the whole of it, her limb (wrist or ankle) or chin is not covered. As regards that, the majority of the scholars are of the opinion that such a person has no sallaah and will have to pay it back. The evidence for that is what Aisha (RA) narrated. She said that the Prophet (peace and blessings be upon him) said;
"Allaah would not accept the sallah of a matured woman except she prays with Khimar (a veil covering her head)" (Ahmad).
Umm Salamah was asked about type of cloth a woman can pray in. she responded by saying "A veil covering her head and wide outer garment that can cover her two feet". (Muwatta)
Also, Imam Ahmad was asked: How many pieces of clothes can a woman pray in? He responded by saying "A minimum of an outer garment (that is wide) and a head-cover (Khimar). Imam Shafi said: "It is obligatory upon a matured woman to cover the whole of her body except the two hands and the face". He also said, the whole of the woman is nakedness except the palm of the hand and her face. The back of her feet is also her nakedness. If what is between the navel and the knee of a man is unveiled while in Sallah and if any part of the body of a woman (except her face and the hands), her hair (small or plenty); both of them will repay the Sallah" (AI Ummu)