Item randomly disappearing from inventory.

I was just mining gravel underwater with my shovel (It was unbreakable since it was a vote shovel from an SMP server) then I saw it come out of my inventory (I didn't press Q) it started floating then it just vanished (No clearlag or anything like that) I looked around the whole area and it wasn't there. So 2 issues. 1) Item ejecting itself out of my inventory. 2) Item ejected vanished. And it wasn't a visual glitch, I tried relogging and it wasn't there.

(No visuals as the item is gone)