When I was 16 I snagged my left leg while dirt bike riding. My knee popped out of joint and I struggled with a lot of pain to get it back. After several unsuccesful tries, I discovered that if I squatted down fully and placed all my weight on the right leg, I was able to straighten the left leg and the knee went back into place with very minor pain. As far back as I can remember both my knees made the "popping" sound (previously described here) when I squatted down fully ... never thought much of it. After the dirt bike incident, I noticed that the left knee no longer made the popping sound ? In my late 20's the left knee began to give me issues, popping out of place maybe once a month. It always went back into place easily by doing my little ritual of squatting fully, placing all my weight on the right leg and slowly straightening the left. By the time I got into my 30's it was coming out of place about once a week now but always went back in with my little ritual. By the time I reached my 40's, it was popping out of joint almost dailey, sometimes by simply catching my toe on the blanket when I got into bed at night. Again, my little ritual always got it back into position with no pain and no problems but I figured it only a matter of time and I would have to address the issue medically.
Then one day when I was 48, I was carrying more luggage than I should have up a staircase to a motel room while on vacation. The left knee popped out again and I collapsed in pain. This time it really hurt bad and refused to go back into place with my little ritual. I couldn't straighten my leg nor put weight on it. I spent the first 3 days of my vacation in the motel room with ice bags on the knee. Eventually, I could hobble around a bit but kept icing it the whole time I was on vacation.
Now for the good part .... At the end of the week we got home and I stayed in bed mostly and kept icing it. After 2 or 3 more weeks, I noticed that I could actually straighten the leg and put weight on it again.
That was 15 years ago and the knee has never popped out of joint since then. It also started making the popping noise when compressed in a full squat like it used to decades before. I don't know how I injured it on that staircase years ago, nor how it healed itself back to 100%. All I know is that I'm mindfull of not abusing it like I used to when I was young