^ That dude reminds me of a science teacher I once had.
But I smoked the first time ever and it had crack in it... They didn't tell me until after the second bowl and I refused more, then they told me they were kidding. So after a couple more bowl packs the session was over and wthey told me there was indeed cocaine inside aswell.
This led to panic attacks related to my heart rate, debating calling an ambulance, etc.
Other than that, a few friends and I went off to smoke during highschool lunch and it started burning, and it fucking smelled like Mr.Clean or something similar so I passed that one up. A few hours later one of them went into the hospital because he started getting tunnel vision, headaches and nausea. Fun stuff.
All other times, I've laced my own.