Final answer:
George falsely believes Myrtle is having an affair with Tom Buchanan, a crucial misunderstanding in F. Scott Fitzgerald's 'The Great Gatsby'.
Step-by-step explanation:
At the time of his death, George mistakenly believes that Myrtle is having an affair with Tom Buchanan. This tragic misunderstanding is one of the key plot points in F. Scott Fitzgerald's novel The Great Gatsby, illustrating the complex interplay of relationships and mistaken perceptions among the characters. George is led to this incorrect conclusion by a series of events and assumptions, ultimately leading to a series of tragic events.