I've always considered bad trips to be a waste of good drugs. I had a few dodgy experiences in the early days but I haven't had anything I'd call a bad trip in years.
A few years back, i did blatently overdo it on MDPV. I guestimated a load of 10-15mg bumps and just generally left them out with a load of friends round on a help yourself but be careful basis. Completely ignoring my own advice I sniffed 8 of these bumps in a period of about 6 hours. Later that evening my body started to become kinda jittery, sorta nervous looking and I pretty much lost much of my ability to speek. This wouldn't normally bother me too much if I was equally wrecked in my head but, although I wouldn't say i was on top form mentally, in my head I was more or less completely straight. Essentially I felt like I was perfectly coherant but I was trapped in a body I had very little real control over and worse still i could barely communicate with anyone. Even when I could manage to just-about explain that my head was pretty normal it came out sounding so forced and wierdly nervous it pretty much appeared to everyone else I was freaking out or something. I wasn't, I was just bored of feeling sober but being stuck in this wired-up, overdosed body.
Worse still, MDPV has this horrible way of supressing ketamine so I couldn't even get myself k-holed and be happy about it all. No, this was just something I was gonna have to sit out, and it lasted for days... I did manage to sleep and the next day i could talk better with it gradually wearing off over the next few days.
Although I would class this as one of my worst ever drug experiences, I'm not sure I'd call it a bad trip. Like I said my head was fine, there was absolutely no freaking, no paranoia no nothing, i knew it would wear off and i knew it was my own fault and i knew that all i could really do was just sit through it however long it took. Really the worst thing was the boredom.
I don't know if someone else in the same situation would have remained so calm-minded, but I've been taking drugs for a long time (about 15 years at the time) and I've just learned to not get freaked by things and to try n laugh off any possible badness. Nevertheless, I haven't touched MDPV since and I wouldn't recommend doing the same thing to anyone else.