#1 Several years ago I trained at Frank Doran Sensei's Redwood City dojo which was between my office in San Jose and my home in San Francisco. On Sundays, I practiced at a small dojo nearer my home, because I didn't want to travel to Redwood City on the weekend and Bob Nadeau Sensei's dojo didn't have Sunday practice. At this time, I had only been studying Aikido for a few months. At this third dojo was a very cocky brown belt. One day, in what should have been a very gentle roll, he forced me into a breakfall, and at the same time pile drove me into the matt. As my shoulder popped, it could be heard across the dojo. I had gotten up from the matt to continue practicing. Both the instructor and the brown belt had horrified looks on their faces. The instructor ordered me to stop practicing. I was partly in shock and partly numb (the pain had not yet begun). My arm was in a sling for a few months after that.
#2 A couple years ago I was working in Singapore and attending a dojo near my corporate apartment. AwYong Sensei used to enjoyed having me Uke when demonstrating how effectively Aikido works on big Americans. One Saturday, he was demonstrating Nikyo and other wrist preassures. My wrists have almost no fleximility. We sat in seiza across from each other as he would apply the preassure. Each time I let out a huge yell as he brought me to the matt (it was so painful). I would rise and he initiated another preassure. For about fifteen minutes this was repeated, my yelps getting louder. Once I glanced over to see the younger children sitting on the sideline in horror. Once the preassure was removed, there was no pain, but the brief duration when I was in the hold was excrutiating and I'm certain that the sounds I made were hideous. My tiny classmates didn't look at me as so threatening after that day.
3 Fast forward to the present. I now practice at a dojo near my home in Denver. Sensei had me uke with a katatori. I was tired and did not know what move he was intending. When he did throw me, I forgot to let go of my katatori. While I was in motion, and without thinking, I executed a one handed hip throw (I studied judo when I was eleven). Both Sensei and I ended up on the matt looking somewhat stunned. I also tore his gi. He prefaced his next demonstration with, "and if Uke proves to be as obstinate as David can be, then you can do this..." and proceded to wipe the matt with me.