Sources A and B differ. Does that mean that one of them is wrong?

On occasion, one may see a question like one of the three below:

1. "Sources A and B give different accounts of the impact of Hitler's rule. Does that mean that one of them is wrong?"
2. "Sources A and B give different accounts of whether Stalin persecuted Jews. Does that mean that one of them is wrong?"
3. "Sources A says that Hitler faked the burning of the Reichstag,  but Source B says that Hitler did not. Does that mean that one of them is wrong?"

The answer is often no. The sources often are just representing different points of view. For example, it may well be Source A is talking about the economic impact of Hitler's rule, while Source B is talking about the political impact. The political impact could be bad but the economic impact could be generally good. Alternatively, Hitler's rule could be good for most Germans, but bad for the Jews; the sources could be talking about impacts on different groups. Finally, Hitler's rule could be largely beneficial before the war started, but disastrous after the war started, and the sources could be talking about different time periods.

However, it is often important to read the sources carefully, as the answer can sometimes be "Yes, one of the sources must be wrong". For example, for question 3 (the burning of the Reichstag), the reality is that either Hitler set fire to the Reichstag on purpose, or he did not. Therefore, answers of "no" are unlikely to get full credit.

The key to answering such question is to determine whether it is a difference in opinion, or a difference in fact. If it is difference in opinion, then neither of them need to be wrong. If it is a difference in fact (and they are both talking about the exact same thing), then one of them has to be wrong.