I was in the world and did a middle mouse click on a block. Then my crosshair disappeared and there was a square in the middle of the screen with inverted colors.
Trying to restart the map and the game: No effect
EDIT: Just recognizing I have a resourcepack installed(faithful). But at first, i had the right crosshair. So maybe it's nevertheless a Minecraft-Bug
Broken resource pack. Please switch to the default resource pack and see if it occurs there.