Lets take an example, lets say you're cruising the highway at 100mph (roughly 140kmh), you press down the clutch and put it in 1'st gear, that is still not a problem, no wear, no damage, all good. But what would happen if you released the clutch?
Option 1: The gearbox splinters into a thousand pieces due to the extreme stress to the cogs.
Option 2: The engine will rev way above its capabilities, with damaged valves as a result, and probably more than that.
Option 3: The friction of the engine will lock up the tyres and you'll most certainly crash the car.
Option 4: The clutch will slip when released, and make the car slow down.
option 1 is the most likely to occur if you release the clutch fast, if you release it more slowly, option 2 is most likely.
option 3 will only occur in a car with a big engine, and rear wheel drive, something like the BMW 750 e32 or similar.
option 4 will only occur if your clutch is worn out.