Should the throwout bearing fail, you'll immediately notice serious issues with the transmission. First off, you'll often come across difficulties when trying to depress the clutch pedal. Soon enough, it might elevate to a point where you can't press the clutch pedal or shift into gears at all. Before that happens, you'll experience problems like gearbox shifting hard. Or, being able to see and hear it grind and screech its way through the gears. If you continue to put more strain on the already-bad throwout bearings, it might even result in complete clutch failure.