I ride in the forest a lot and after a storm that has felled trees, there are always loads left that are still falling. Yes, they look safe propped up on the other trees, but they are not. I have taken a chance and ridden under one for weeks and weeks. But then the next time I come, it's down! You can be sure that it didn't slide gracefully to the floor, but went in one almighty crash as the last branch holding it up finally failed. It could have chosen to go when I was underneath it.
There was another one that for months and months was propped up across the off-piste trail I used. I had cut off the lower branches so that I could ride underneath it. I had to bend low over the bars to pass under safely. But one day I snagged my backpack on it and got squashed down into the suspension and couldn't move! Obviously, the tree had continued to fall and the gap eventually had got too small for me. I tried wriggling about, to no avail. I was about to remove my backpack when I had a brainwave! I used my dropper release to drop the saddle and I was free! But all my wriggling about could have brought the tree all the way down. My riding buddies thought it was funny, but I considered it a lucky escape.
And I had to stitch my backpack back together again!
The next time I went that way, I cut a trail around it. The tree later fell and I still use the go-around.