I think one of the worst students I had, unfortunately started off at the same time I did, however he was younger by three years. Thus, when I hit green belt (Its required to Sempai if you want to continue on in the dojo I was in.) I ended up his Sempai even though he was a rank behind me. He did everything in his power to try to mess me up, including telling the class all the techniques I was showing were incorrect, trying to correct my counting, etc, etc...
It was very disrespectful. He ended up stopping once the other students got bored with his childishness, lol. Other then that, most of the students and people I pracitced with were good.
*Note : Although at one point I was in a Tournament in San Diego, and someone tried to walk up and take our weapons, lol. I thought it was the wierdest thing. It was a Dojo from Spain I believe, it was a few years ago and we were all horrified because none of us knew any spanish, so I screamed out. "PALO ENFUAGO!" (I think thats how it is spelled) Which basically is. "Stick on Fire!" and the guy drops our weapons in the case and bolts, lol.
Thats about it. Other then psychotic children who blocked with their face.
While everything changes, nothing is truly lost.