Is it considered cheating in an exam for a student to revise their answer based on a result they saw on a classmate's calculator, and then re-solve the problem themselves to find the correct answer?

What happened is not cheating, and there is no harm in it. If a person inadvertently comes across the answer to a question and writes it down, there is no harm in it for him. It is even more so if he did not copy the answer but noticed a calculation error and re-did it unintentionally. Sheikh Ibn 'Uthaymeen was asked about a student who unintentionally overhears the answer to a question from those around him, and he permitted it, because the student did not intentionally commit a sin.