I had a few in high school, one in particular was very memorable. I was swimming laps around 5am with my high school team, and I felt what some people call an "aura," that is, a feeling or premonition that something is going to go wrong soon.

I hauled ass out of the pool, and grabbed a towel when it hit me. I started crying and shaking uncontrollably, and a very heavy weight was on my chest. I particularly remember imagining my father dying in a car crash, or somehow, and the police were talking to me. This wasn't the case, but it felt so strong and tremendously overwhelming. Lasted for about 10 minutes, and then it stopped.

Never got one from smoking weed. Although I imagine some panic attacks could be triggered by the baroreceptors in the blood stream releasing similar chemicals responisble for panic attacks, due to the common side effects of increased blood pressure and pulse from smoking too much pot, particularly colliding with a personality that is favorable to panic attacks.