Dragging volume slider past 100% can trigger the mute toggle
The mute toggle is to the right of the volume slider. When you drag the volume slider to the right past 100%, it is natural for the mouse pointer to end up over the mute toggle while the mouse button is still depressed. When the mouse button is released, this is mistakenly registered by the mute toggle as well, which causes the volume to be muted. So the result of trying to increase the volume is that the volume is muted, the exact opposite of what is intended. To avoid this it is necessary to very carefully make sure the mouse button is released at the exact end of the volume slider rather than overshooting, which is contrary to how most GUIs work.