My rear has been nailed, screwed and penetrated far too many times!
I had one on the car and when they swapped out the tyre they handed me the item they found in the tyre.
It was one of those square steel bars that joins door handles together. About 3" long and must be ½" square.
It was a big hole in the tyre and it did go down awful fast.
The problem is that the front tyre rolls over the screw/nail and stands it up.
If yer unlucky the rear then rolls onto it whilst it's still upright and it penetrates.
The same 4:1 ratio applies to cars too.