727 rear band pin fell out!

Hi all!
Last week a really wierd thing happened to me. Rear band pin fell out of my 727 so I lost reverse and like 30% of oil from the gearbox. All underbody and rear axle are covered with trans fluid...
Can anyone tell me if this just happenes sometimes or someone before me screwed something up?
I found the pin on the sreeet a day later near giant trans fluid stain. Pin had just one o ring on it, one was missing.
After removing the pan I found all the missing parts plus detend ball from the gear lever out of valve body.
Turned out that gear lever was very loose so detent ball fell out (also this is why I had problems starting the car in park). I spot welded the selector piece to tubular part fixing detent ball problem.

My question is: did any of you guys loose a pin???
Coud this have happened cause I went from 3rd gear to 2nd to help myself on the braking (emergency stop)?

I already put everything together, just waiting for new filter so I can put new oil in the trans and get the car back on the road.

Aslo wanted to make sure about adjusting rear band lever.
How many Nm should I use how many turns back with 4.2 lever?
I used 8Nm and 2 turns back, is that correct?