Getting into a chair earlier this week, I put weight on my arms while lowering myself: a routine process that I've done tens of thousands of times over the decades.
This time I heard a sort of 'sitting-on-an-egg-carton' sound in my right shoulder, accompanied by a complicated set of signals from the neurons in that zone. The nerve endings were mostly saying 'OUCH!!!'
Since then, I've had to be very careful about rotating the right shoulder in certain directions: mostly front and up. It's a nuisance, but manageable.
The cause of this frustration is obvious. I'd forgotten that using the mouse while resting my right elbow on my desk's shelf is convenient at the moment: and a very bad idea in the long run.
There's a moral there, but never mind.
I'm keeping the shelf retracted, learning to use my left arm for some procedures, and trying to remember that exercise is important. Based on previous experience, I should have normal function back in the right shoulder in a week or three.
This situation, needing trifocals and task glasses to see properly, and noticeably less tolerance for late night work, are all reminders that I'm not a 40-year-old kid any more. On the other hand, this sure beats the alternative.