Safety is paramount, no use breaking everyone, there will be no one to practice with.
Just last night I was teaching class. Whilst everyone was practicing a particular technique, I grabbed the dojo-cho (25 yr + aikido practicioner) to one side to try something I thought I would teach next. It didn't work very well so I gave up on the idea. A few minutes later he came back to me with an idea and we went over to a corner to work on it. First attempt was OK, second attempt I threw him, ukemi was a bit tight, he managed to turn his head before driving his shoulder into the mat. Needless to say he was out of action immediately. A visit to the doctor and casualty this morning, very fortunately not broken, despite early thoughts from the doc, but enough ligament damage to keep him out for a few weeks
We weren't messing about, we weren't going hell for leather, we were just trying something out that did what we expected it to do, just a little tighter than anticipated. On reflection, I don't think either of us would have done anything differently, we have to accept that these things happen in our practice from time to time, but that doesn't negate the need to take care of our partners at all times.
Bryan (feeling very down)