I once dislocated my collar bone (not aiki related; falling drunk off a pedal bike). I didn't notice it much at the time, I thought it was a dislocated shoulder or really serious bruising. However it really incapacitated my upper body for several weeks. Using my back, twisting or even using my other arm, was painful.
It made me realise that the potential to get seriously injured in a fight can massively limit your ability to defend yourself. I couldn't do Aikido at the time, but I used to spend a lot of time down the gym kicking a punch bag. Though I would never use a kick in most situations, I always practise them 'cos you never know when the situation is going to be very different to what you'd expect - probably a good lesson to cross-train.