O'Sensei killed one student and seriously injured another in the same seminar. All the deaths I know of are from the original honbu dojo. I think at least two or three were from shihonage. The most serious injury I've seen in my organization was caused by hakama entanglement (totally jacked up his leg, torn something or other)...although I am quite cognizant of the high injury level in aikido. Even though 99% of aikido is cooperative these injuries still occur, usually due to a lack of control on the part of nage b/c they halfway learned technique, to include ukemi. See how I brought that back to ukemi? Full circle, baby, oh yea!
A shidoin once told me that at a certain level of Aikido "you know just enough to be dangerous"