Chagigah 20 - Many levels of purity
On the Holidays, there is a special requirement to purify oneself, in general, and also because one is going to visit the Temple, eat sacrifices, and also eat foods of second tithe, which all require special attention to purity.
However, it is not enough to go to the mikvah, but one also needs to keep in mind why he is going there, and moreover, keep this in mind consistently. For example, there were people who would eat their regular food only in the state of purity. And yet, when such a person goes into a mikvah, it is only good for regular foods. He would not be allowed to eat the second tithe. Even though it is the same mikvah and the same person, but it is not the same level of attention. Thus, while being "pure for regular food" he may have missed some subtle source of impurity which would prohibit him from eating second tithe - just because he was not watching for it. All in all, there are five such levels: regular foods, second tithe, priestly portion, sacrificial foods, and finally the ashes of the red heifer .
Examples of these rulings: two women accidentally exchanged their clothing, and even though they each watched them to keep them in the state of purity, still Rabbi Akiva declared both sets of clothing impure. Why? As soon as the first woman realized that her clothing was on her friend, of whom she was not sure if she knew all the laws - she lost concentration on her clothing, and this moment was enough to distract her, and any time one does not specifically watches the purity, it is lost.
Another one: a woman asked Rabbi Ishmael a question - she was pure while weaving a garment, but she did not have the intention to watch the garment. In the course of queries she recalled that in fact in the beginning of weaving, before a significant piece of clothing was formed, she may have made some threads impure. Said Rabbi Ishmael, "How great are the words of the Sages, who said that purity requires concentration."
Art: Women Weaving By Enoch Wood Perry