Sadly we can also hurt without knowing it. I once joked, in public, to a journalist, 'still misleading the public'. I thought it was funny.( I quite often greeted fellow teachers with 'still misleading/corrupting the youth) A few years later he told me how he has not wanted to see or talk to me since, and how upset he was. I of course apologised , and thanked him for finally letting me know. I misjudged the ocassion, and was fortunate to get a second chance to rectify it.