On Stack Overflow, when I look on my account it currently shows 1,040 reputation. However the graph shows 100 more reputation, sitting at 1,140.
Is there some sort of bug or error happening to cause this? How can I fix this?
Check your reputation history (make sure "show deleted posts" is checked). The most-likely cause of a 100-point discrepancy is that you had a post deleted as spam or rude/abusive. In the case of these deletions, the reputation number and graph do not match. Nobody else (other than moderators) can see those 100-point penalties in your history; SE made the conscious decision to not embarrass people like that. Except that there's this mismatch on your profile page, so it's kind of implied.