well, I've had my share of students who have a lack of control in one way or another.
People who just hit too hard - I've been dealing with them almost since I began training. When I was a beginner, I learned how to block - and I was quick, too - but I had a hard time moving aside. So, whether I blocked something or missed it, I took the hit. I was also determined (maybe a bit arrogant?), and I just sucked it up and learned how to take it. As a result, senseis knew that I could take a hit well, and so they parternered me with the guys who needed help with their self-control... I'm glad actually, it helped me take hits better, and I got better focus and control myself, hitting just the tip of their noses...
The other kind of lack of self-control is, of course, the student that just can't pay attention to anything for more than a second and/or without interjecting their own opinion and experiences. Plenty of those, and man can they be trying.
One guy I knew was a junior shodan (meaning he was under 16). I had just passed my shodan test and he had passed his about two years earlier, but had stopped training regularly after that. He was used to coming in every so often and giving advice to kids who were practicing, and joking around with some of the other senior students.
Well, when he started attending class again, he was kind of a goof-off. The class was sparring once, and he's trying to talk socially with me in the middle of our fight...! Making jokes, saying stuff like, "oh, that was a good punch," or "too slow! try it like this next time," and he would constantly drop his hands and go into "casual" mode after each contact.
Luckily for me (not for him so much), we went into continuous sparring (sparring+takedowns+grappling) after that. I trapped his front leg and just pushed him over, then grabbed his head.
...He never wanted to spar with me after that. go figure.
Fortunately I've had some great people to work with too. Guys who are miles ahead of you in their training but really understand how to teach you something without beating up on you or losing on purpose. Guys who show soo much genuine appreciation when you show them some fine point that they'd never even heard about.
There is one guy at my dojo now who is just got his brown belt. He's extreemly personable. Talks all the time in a genuine, friendly way; a big guy who's language is sort of...strong? yea, that's a good way to put it. But he understands the concept of respect as it relates to karate. he knows and understands many techniques better than other students who outrank him, but he is so patient and still makes it obvious that he's learning from them. In the dojo, and most of the time outside, he calls his senseis "sir" - even though a lot of them are half his age. He's one of those that you're just proud to call someone you train with.