don't always know how something that seems that harmless could hurt someone else.
My wife, for example, lost her mom to cancer a few years ago and grieved it pretty hard over the ensuing months. If someone threw a "Your mom!" joke at her without any harm or offense intended, it likely would have momentarily sent my wife over the edge. Grieving and battling depression at that point of her life didn't have her processing things that well.
A rare exception, but one that I choose to give people the benefit of the doubt on.