STEPS TO REPRODUCE
1. Select contiguous selection tool
2. Use [Alt] to use subtract
3. Don't move the mouse
The minus symbol stays on the screen
The minus symbol should be removed once the hotkey has stopped being pressed
Windows: 7 Professional x64
If you move the mouse the minus sign will disappear, but if you just hit [Alt] it will stay there until you move the mouse. If you don't move the mouse and hit [Shift] it will stay as the minus sign and doesn't change to the plus sign.
It's a tiny bug but it can be confusing as visually it looks like [Alt] is a toggling subtract.
*** This bug has been marked as a duplicate of bug 410532 ***