The white circle that appears when you hold left click on something (the upgrade button, for example) will stay if you right click while the circle is doing the animation.
This small bug can happen everywhere you can hold left click on something so that it does something. (upgrade all the same towers button, open multiple boxes... the list goes on)
It can manually be removed by the player, but in some conditions. (in the main menu, you have to leave the said place (shop or inventory) and come back. In a level, you have to exit the level by ending it or just by going to the main menu, and come back. In the custom map editor, you have to leave the editor and come back.)