The Eye Dropper Tool Bug

Now this is weird. I am sure many of us are facing this. it goes like this

Select the eye dropper tool

pick a color 

Fill it wherever you want 

Switch to another tool. Say the "Pick' Tool 

and here comes a square box on the document. A small Square box which refuses to disappear come what may. It does not harm the document. It does not print nor does it come in the jpg if you export to JPG. But it is visible and it is irritating. 

Ah yes it vanishes once you close the document and reopen. 

Now is this a bug or whatever. But man it gets on my nerves.