One common error in reading primary sources is that people don't report what's normal in their lives. If someone writes a letter saying, "I was up at six this morning." and you want to know what time she normally got up -- this is evidence that it wasn't six. At least, normally.
Unless for some reason she was being unusually scrupulous about reporting her days. Ah, the delights of reading between the lines.
You get more dramatic effects when you read reports of how horrific a certain noble was. Not only do you have deal with the agenda of the person writing about him, which would probably give motives to blacken him, you also have to deal with the issue that the noble (because he is being written about) is probably an outlier.
Or the story of the ancien regime French nobleman whose son said that he had heard his father intended to marry him to a certain mademoiselle and was it true -- and who retorted that his son should mind his own business. Not the sort of incident that would occur in a society with more freedom for the young people to make their own matches, but that the story was told at all is evidence that the nobleman had gone -- a little far.
And I have read people who took the comment about Henry VII to the midwife about saving his son over his wife because wives were easily gotten -- as evidence of how men in general thought. I will pass lightly over the fact that it appears he didn't say it, because it does appear that the midwife invented it after the fact to describe his attitude. But she made it up or repeated it because it was scandalous. And indeed, two princesses were so bold to refuse his suit explicitly on the grounds that they thought marrying him hazardous to their health.