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
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Broken resource pack. Please switch to the default resource pack and see if it occurs there.
it doesn't
Updated the report. Maybe it is nevertheless a minecraft-bug.

This is an issue with your resourcepack
This is an issue with Minecraft... 90% of my resourcepacks have this square..
This is how I managed to do it:
Kill yourself and pick up an item, and the square will appear.
Right after the death the crosshair is normal. But picking up an item bugges the hell out of it.