I took a friend rolling for his first time to an EDM show recently and made sure to tell him that some people have anxious come-ups. I have never had one myself, but his was pretty bad. He explained it to me and it reminded me of when I became anxious on LSD.
He said thoughts were going through his head like, "Did he make this drug himself and is testing it on me?" and at one point he heard my friend ask how much he took, which translated to him thinking he was overdosing and dying. At this point, he had his eyes closed and was resting in the car with all of us, so he was safe, but he felt like he was dying.
He said the only thing that kept him sane was the fact that I told him before that any bad thoughts that occur during your come up are all mental and it's part of the anxiety. He kept repeating "It's all mental, it's all mental" to himself in his head.
I went through this on LSD and thought I would never come out of the acid trip. This lasted for 5-10 minutes for him but he said it felt like an hour.
So just make sure if you're with someone who is trying MDMA for their first time that they are prepared for a potentially bad come-up.