When you mine blocks that have at least one solid or nonsolid block above them, the resulting item has a chance of flying up through the block.
To reproduce:
go into a new world.
Find a cave near the surface or even a tree
mine the blocks above you
After a while, come outside and you should see items littering the ground above your cave.
Duplicate of MC-47650 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.