You draw the line where ever you need it. I make jokes about all sorts of stuff all of the time which I understand and the people I am saying them to understand. I think that the only time I have ever really thought I have crossed the line is when I showed this girl who I was pretty interested in a meme that was something along the lines of 'when your neighbours autistic child shits in your garden again so you run them over with your lawnmower'
[insert picture of that fat person from moana]
'What can I say except your welcome?'
Her brother was severely autistic.
Let's just say that she had about a 20 minute rant at me for being a horrible human being and we haven't really spoken since (this was about 2 years ago I think).
But apart from that, I haven't really offended anyone seriously and only say really dark stuff to people I know will take it as a joke and not like she did.