Here are a few different stories, not in any particular order:
1 - We were practicing punch evasions in practice one day and my instructor was getting annoyed because I wasn't really trying to hit him. He planted his feet, put his arms down at his side and positioned his head forward a bit and said "now, hit me." I figured he would move at the last second, but he didn't and I ended up punching him right in the face.
2 - I don't get to do Aikido very often now that I've graduated, but I occasionally drive back to campus to practice and hang out with my friends. During one particular visit, I hadn't seen one specific friend in over a month. When she stepped into the studio and saw me she called out at me, raised her arms and charged at me, full speed. My first reaction was to take her down to the mat, which apparently was not what she had in mind... "Matt!" she yelled, "I just wanted a hug!"
3a - I have Tourette's Syndrome and some of my tics involve snapping my fingers and one-hand-clapping, so my hands are much stronger than what you would expect for someone my size (145 lbs). This tends to surprise people when applying the Yonkyo nerve pinch
3b - I was working on Nikyo with a new girl, teaching her how to go down to one knee and to tap out when it hurt. At the end of class, when it was time to shake hands and thank our partners, I approached her, said "thank you" and shook her hand. Then she fell to one knee and tapped out
3c - Fortunately, a lot of the other students think it's funny that I (accidentally) squeeze so hard and sometimes try to squeeze my hand as hard as they can. I just laugh and steadily increase the pressure until they submit
4 - I was once helping a new girl with one of the kokyunage variations and showed her an example of how with the right balance and momentum it's even possible to only use one arm. I didn't anticipate that my example would be THAT helpful, so I wasn't prepared for her next try and she threw me into a heap on the ground
5 - Sometimes I get a charley horse on the bottom of my foot during seiza, so I usually sit cross legged instead, but on a few occasions I've been able to sit in seiza for several minutes while the instructor was speaking. Unfortunately, almost every time my ankles lock in that position and then I'll either jump up quickly and fall right back down or have to stretch my feet back into their standard standing position before I can get up.