What is the error in the quote "How many cups of coffee must I drink," she asked herself, "in order to study all night for her test?"

The issue with the quote arises from the context of direct versus indirect speech alongside punctuation usage. In this instance, the phrase is a mixture of a question within a statement. When the speaker asks a question and then continues with another thought, punctuation needs to reflect this appropriately.

The phrase "her test?" should have a period instead of the question mark. The question mark is already included in the preceding part of the quote, which negates the need for a second question mark within the entire statement. Therefore, the quotation should conclude with the correct punctuation to maintain clarity and adhere to grammatical conventions, indicating the end of the complete thought.

This punctuation adjustment ensures that the structure of the sentence properly conveys the intended meaning and maintains the flow between the direct question and the subsequent commentary.