Calumny, as you know, is the act of making a remark contrary to the truth that harms the reputation of another.
Suppose a friend asks you for your opinion about a local tradesman. You reply, "That guy? He does shoddy work at a higher price than his competitors."
This is fine, if you happen to know that he does shoddy work at a higher price than his competitors.
But if he doesn't, you've committed the sin of calumny.
Now, what if your friend mispronounced the name of the tradesman, one who happens to be competent and inexpensive, or if your memory of the tradesman's quality and price is faulty through no fault of your own? Suppose you make an honest mistake, and unwittingly tell your friend something that is contrary to the truth that harms the tradesman's reputation. Assuming you aren't culpable for the mistake, then you have not committed calumny.
But... we still have a remark contrary to the truth being spoken. We still have harm to a reputation.
Where'd the calumny go?
Calumny is a sin, which means it's an act chosen by a moral actor. But if you've made an honest mistake, then you have not chosen to make a remark contrary to the truth. If you don't make a choice, you don't commit a sin.
It's not that, objectively speaking, calumny happened but you aren't culpable. Calumny didn't happen.