The only bad trip i've experienced was basically just a panic attack from hell. I probably lost 5 pounds just sweating....
One of the thoughts I had during that 2 hour period was when I noticed a screwdriver on the ground and I thought "I can stab myself with that and this will all end..."
The panic attack was induced when I was left alone...I started to think about EVERY person I had ever wronged and all the bad things i've done. It really got to my head....
The bad trip was very positive for me though...it made me analyze things I had been bottling up, and after the trip was over I felt so fresh and free of the anxieties that had haunted me before. If that makes any sense....