The orthopod who did my ACL graft/meniscectomy told me he worked on a guy who totally blew his medial meniscus going into lotus position and either on aikiweb or aikido-l yrs ago somebody posted they blew a knee out getting up from the mat.
Life is dangerous but beats the alternative
I can beat that...
A few years ago I had just returned from an intensive weekend aikido course, and was lying on the sofa watching TV. My calves were feeling quite tight, so I thought I'd try to massage them as I lay there. I pressed my right heel into the top of the left calf, and all of a sudden there was a "crack" and I felt something give inside my knee. It subsequently swelled up and it was several weeks before I could walk without any discomfort. You certainly don't need strenuous exercise to injure yourself...