Anything that permanently and seriously denigrates the dignity of the significant other. The sort of thing that's a permanent insult or besmirchment, where you are sorry you said it almost before it's fully out of your mouth, no lover would say it, it can't be taken back, it's not part of the present fracas so time won't wash it away, it's one's position on some aspect of the other that is unchangeable or highly unlikely to change, it may as well be a service of process in a divorce.
I've done it. I told my girl that she was badly educated and pig-ignorant about science. She, of course was Magna Cum Laude from Sarah Lawrence. She did forgive me, but it took a long time.
Keep the unkind cuts to remarks about the current fracas -- that all gets washed away with time. Fight over, insults gone. Anything that is purely personal and not changeable is ultra hazardous to say (unless you want a divorce).