I noticed that when you throw out items directly from your inventory on the ground, they have a chance of dissapearing. It only happens if you are in creative, and only if it is your inventory. I've tested it with a chest and in survival.

It can be pretty annoying when you need to throw out many items at once, for testing things and such. (and you still want the items to be there)

How to test it yourselves:

1. Go in a world in craetive.
2. Fill your inventory with an item. (just like in the attached files)
3. Throw them all out in one single go. (using ctrl and q)
4. Then go and pick all the items back up. This is when you should see that about half of the items has dissapeared!

I tested it with and without OptiFine, and it got the exact same results both times.