If you are in gamemode 1 and your inventory is full, so normaly would not be able to pick up items anymore, you can still pick up items.
The big!!! problem is that these picked up items will vanish and be gone.

What I also discovered is that these items will still somehow be stored in a overflow cache.
For example if your inventory is completely full and you pick up one or even more stacks of stone, the stone will be gone. You picked it up but can't access it because your inventory slots are already taken for other items.
But if you clear one inventory space and do middle click on stone then you will get 64x stones and these are the stones that you picked up.
Lets say your inventory was full and you picked up 4 stacks of stone.
Do the steps described above, so clear one or more inventory space and middle click on a stone block. you will get 64 stone - delete them - do middle click on stone again - you will get 64 stones. Do that 4 times and every time you will 64 stone. Howerver the 5th time you will only get 1 stone.
(The thing with the 64 items by middle click can't be reproduced every time. Just tested it once more in a fresh world and didn't work. Just as a addition so you don't wonder)

So my explanation is that you will get the picked up items back if you do so.
The are still stored somewhere.

Nevertheless this can't be intended because first this is really annoying and second you will defenitely loose items you can't pick up with middle click or are modified.
The fix is that you can't put up items if your inventory space is full

This is a huge bug and must be fixed as fast as possible.
You can't afford releasing 1.8 with this bug!
Mods put this on high priority please - everyone vote up please

@Mojang: There are still other high priority bugs you do not even pay attention to.
You said "vote up bugs you want to get fixed" - there are bugs with hundrets of votes you do a sh't!

Want an example? The freakin dark oak wood bug where you can destroy every block even bedrock and barrier blocks.
Thats in the game since 1.7!!! Goddamn pay attention to important bugs!