trying moving in any direction continuously, and while holding the movement key also press 6 on your keyboard. This should bring the inventory cursor to slot 6 like the rest of them, but it doesn't. This bug has been in the game for a very long time (at least since 1.5 or 1.6).
EDIT: I was mistaken about moving in any direction, it only happens when you hold down the forwards key.
I've seen this issue on a variety of computers, however it's never the same key. On my computer it's the 2 key.