When I try to do a single click with the left mouse button, it is sometimes treated as two separate clicks. Specfically, if I don't release the left mouse button immediately after I press it, mousedown and mouseup events (or however it's coded) are treated as two separate clicks. This can be annoying when doing combat where you have to click to target something, or when you're trying to nurture a sylesti and the "double-click" makes you accidentally pick the wrong option.

If this is happening randomly - your mouse button itself may be going bad or some dust may have gotten lodged into between the button and the sensor.