A huge pet peeve of mine:

The worst insult you can give someone is to respond to something they say with a dismissive, “Whatever…”

You’re telling them that they could say anything and it would make no difference to you; it implies you think they and their ideas are not worthy of your attention, time, or followup.  Ultimately, it sends the message that you want nothing to do with that person.

Don’t say it unless you mean it.  Bad karma.