The cursor of your Minecraft first person view will change when items are inside of your hotbar, displaying a cursor bound to a graphical change found in the menu of "Super Secret Settings". This does not occur when you have no items in your hotbar, nor when you have items only inside of your inventory slots. I am not aware if this is a mere change or a bug, and would appreciate some feedback as soon as possible.
Hope to hear back soon,
Karl aka Titanic
It does also not occur when you press a number on your keypad while you hover over the block you want with your mouse.