What errors reveal about a historical source

How many major errors can a source contain before you discard it?

There is no fixed limit; errors can expose a source's perspective and should be tested against procedures and primary records.

A book's pattern of errors can itself be evidence, so a fixed number of mistakes need not justify discarding it. The example is a Polish account of a submarine loss whose description conflicts with British inquiry procedures, recognition signals, distress practice, and the ruthless escort logic around a convoy. Comparing the claim with the court-of-inquiry record and operational procedure helps reconstruct a more plausible event.