Trigger warning: sexual assault:
This isn't a subtle red flag, but it might seem like one: if you admit to doing something bad, they will equate it to something significantly worse that they did.
In my case, I had a tendency towards being flirty, online, from a very long way away, with our mutual friends. One of the friend group started abusing me really awfully over that.
I had gotten used to being apologetic to my narcissist, so I apologized *for talking to that group of people*. My narcissist and one of his friends actually said, "I have something like that. I'm a convicted rapist, a couple of times over." Two of these guys said that, as if serial rape were equivalent to inappropriate flirting.