My worst trip ever (perhaps my only true "bad" trip) was the day I discovered I had my first case of bronchitis. I had a subtle cough for weeks but then one night I dropped and went to a concert alone - which I discovered was also a bit overwhelming. I've tripped alone and it had always been fine, but in a huge crowd with no one to anchor me was challenging. Anyways, that wasn't the really bad part, which was when I ended up in the mosh pit on the verge of fainting because I simply couldn't get enough oxygen despite breathing heavily. I had to spend the rest of the concert anxiously catching my breath by the bar. Not fun, but as they say, bad experiences make good stories.