Occasional glitch in the Paint tool?! This weird glitch occurs when you try to copy something, where that blinking image cursor is out of bounds of the copy position after you press 'A'...

Made even worse that you can't undo the mistake, compared to if it wasn't glitched.

After you copied it, you should push the B button！

this happens when you switch tools without hitting b after copying. if you have the copy stuck on screen you can use the fill tool anywhere to get rid of it. just fill an area with the clear color and the glitch will stop.