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Julie is a real hypochondriac when her stomach
hurts, she is certain that she has a bleeding ulcer, and if she has a backache,
she believes that she has cancer of the spine.
The problem occurs between hypochondriac and when. Julie is a real hypochondriac is a main clause, so it can stand alone as a complete sentence. When her stomach hurts, she is certain ... begins the next complete sentence.
Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after hypochondriac and then capitalize the W in when.