I knew someone who dislocated her shoulder often while sleeping. If I remember right, she's dislocated it 17 times or some ridiculous number.

this happens to me quite frequently. i grew up playing years of baseball and my throwing shoulder is a bit loose as it is. combined with the way i fall asleep (on my stomach with my forearm under my pillow) i tend to dislocate or severely strain it pretty regularly

Isn't it easier for it to happen again once it's happened at least once?

For me it's often a foot sprain, but exercising has made it less frequent.

The neck is quite vulnerable during sleep and could lead to neck muscle injuries that leave one with a stiff neck for days.

Are you hypermobile?

My son is and once subluxed his shoulder while running