If someone says something you find offensive? Watch
If it’s an insult towards me, I’m usually so taken aback that I end up not saying anything and I guess ‘letting it slide’. Especially if it’s a stranger. If it’s a friend, I am more likely to say something, although my friends don’t say horrible things (unless it’s a joke) because well, they’re my friends.
severity (small remarks I don't want to waste the oxygen in arguing)
whether it was justified or not (some offensive things could be justified if it was a small specific point)
if it was within a professional workplace/event (time and place for everything including reactions)