1) The people in the class are there to train and will not mind the pain as long as you don't actually harm them. Yes accidents do happen but if the decent people they will understand. In one session I managed to hurt someones arm and someone else dumped a person on their shoulder and hurt there collar bone (both grappling). Both were accidents and both of the injured watched the rest of the lesson and then trained next time with no issues what so ever!
2) If your having problems with a technique and you mess up your obviously doing well as you've spotted a chance to use it. If it's not a contest just get your partner to let you try again at a slower pace and keep doing that until you get it.

And trust me you will get it.