Succah 6b: By Torah law, a chatzitzah only invalidates if it covers most of the body and the person does not want it there. But the Sages forbid the case when it covers most of the body and he does not mind, as well as the case when it covers the minority of the body and he does mind.
סוכה ו ע”ב: דאמר רבי יצחק דבר תורה, רובו ומקפיד עליו ־ חוצץ, ושאינו מקפיד עליו ־ אינו חוצץ. וגזרו על רובו שאינו מקפיד משום רובו המקפיד, ועל מיעוטו המקפיד משום רובו המקפיד.
Rabbi Yisroel Reisman told the following story to illustrate how people don’t always tell the rav the whole story. An experienced rav knows he may need to ask questions to get the information needed to answer the shailah.
A young man who learned in the kollel of Torah Vodaas came to Rabbi Reisman and said, “My wife went to the mikveh last night, and then in the morning she realized that she had missed removing nail polish from one nail.”
Certainly women do not want to be seen with just one nail polished, so this would seem to be a real chatzitzah. Rabbi Reisman couldn’t believe that the woman did not catch such a glaring mistake during her preparations for the mikveh, so he pressed the husband, “How could that be?”
“Well,” he said, “she was in a cosmetics store and wanted to try out a new color, but didn’t want to mess up her fingers, so she took off her shoe and sock and tried it on one of her toenails, and then a week later, when preparing for the mikveh, she forgot about it.”
“Ah, so it was the toe, not the finger. Does your wife wear sandals or open-toed shoes?”
“No,” he said, “her toes are always covered in public.”
“In that case,” said Rabbi Reisman, “her tevilah was fine. The nail polish was not a chatzitzah because she didn’t mind having it there.”
Source: Shiur on Yoreh Deah 198:1
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