What I expect to happen: when I have enough place in my backpack, I expect that I will obtain every piece of resources. I have not experienced problems up to snapshots of 1.7 - but now they exist.
What actually happened: when I collected gravel (either with hand or with a shovel), not each piece of it appeared ready to be collected. I hit the gravel block but after a while it can disappear completely. Nothing was added to my backpack.
A very similar problem was with wheat. I harvested fully grown plants but did not got as many seeds as I should get (for example, 10 plants of wheat gave me only 8 seeds).
How to reproduce the bug? It is not simple because the bug does not happen every time. Try to use a computer with not very strong hardware (actually, it was tested on a laptop, newly bought but not so well equipped as some desktop computers). Try to collect many items of the same kind at once.
It looks like some new procedures are so resource-demanding that some items are just omitted. It should never happen, slowing or even stopping the game for a second would be better solution. Then resources would not disappeared at least.
Wheat is dropping 0-3 seeds - bad luck.
Gravel is droping either gravel or flint.